In practice, just shrugging and not doing anything about it, is a very common response! Of course, it's not quite that simple in reality. We would first identify the cause of those dips and spikes, as that will suggest an appropriate treatment. Commonly, EQ is NOT an appropriate treatment! In the case of a dip caused by a cancellation for example, EQ is typically not an effective treatment because EQ boosting simply increases the amount of energy equally for both the direct sound and the reflections causing the cancellation, resulting in a net gain of very little or nothing at all. Absorption or the re-direction (diffusion) of those cancelling reflections would be very substantially more effective but, in the case of the reflections being caused by say the mixing console, we obviously can't cover the console in absorber or diffuser panels. There's really not much option other than just shrugging and doing nothing about it! Even in the case of spikes, EQ is sometimes no more than a band-aid rather than a cure. Ideally, we need to think in terms of the time domain itself, rather than just the timing of reflections and the resultant affect on freq response. If a spike is caused by some sort of resonance (or ringing) for example, then we not only have some amount of signal summing but also a substantial increase in the duration of that ringing freq, IE. Not just a freq problem but a time/duration problem. Just using EQ as the treatment may lower the average amount of energy at a particular freq, to the point where the response looks flat but it hasn't addressed the time/duration issue. In other words, to counteract the increase in total energy due to the longer duration of that energy at a particular freq, we've reduced the total energy so our freq response looks flat but if we were to take a snapshot of a particular instant then that freq would have significantly less energy (be a dip). That's why a "waterfall" plot is a useful measurement tool, in addition to just a standard freq response plot. Absorption would probably be the best solution here, but again, applying absorption maybe a practical impossibility.
Shrugging and doing nothing about it is the typical option for problems above about 800Hz, although there shouldn't be too many really serious problems due to the initial design, construction and treatment. Higher freqs are particularly sensitive to very small changes in position. What may have been a 5dB dip at say 1.5kHz may become a 5dB boost, just by moving the measurement mic an inch or two. We obviously can't tune a listening point to just a square inch. Even if we could position our head that accurately all the time, we have two ears which are more than an inch apart! How do you treat that with EQ?
From all this, a few things should be apparent: 1. Acoustics is one of those audio rabbit hole areas; the more you investigate, the deeper you realise the hole goes! 2. EQ is both a blunt and frequently ineffective acoustic treatment tool. 3. A flat freq response is only part of the picture. It's entirely possible that a "flat" mix/mastering room is neither particularly accurate, particularly neutral nor conducive to producing quality audio, even if creating a "flat" room were attainable in the first place!
Ah, but this our biggest point of disagreement! There are two elements to my disagreement: The first, I've addressed before and in more detail above. There have been some fairly extreme solutions to the issue of attaining an accurate/neutral response while avoiding the even worse pitfalls of an anechoic chamber, here's an example of such an extreme mastering room solution:
While covering almost every inch of the studio in quadratic diffusers probably gives an amazing result, the reflective surface closest to the mastering engineer (and directly between him and the monitors), the console, is obviously not covered in quadratic diffusers. So however flat/neutral/accurate this mastering suite is, it's still probably some way off "ideal". This mastering suite is obviously substantially different from the pictures you previously linked to of other mastering studios and would presumably sound at least somewhat different.
The second element of my disagreement is subjectivity, the personal preference/s of the mastering engineer. Although not a bass-head, I do like a little more bass than average and my tendency would therefore be to add a little too much bass to my masters. I sometimes counter this by adding a few dB of bass to my b-chain when mixing or mastering. Some other engineers add even more, most a little less. Obviously, this is all subjective rather than objective. It's a subjective observation that I tend to prefer a little more bass than others, a subjective determination of how much and a subjective determination of whether to counter it with just personal awareness or by actually altering my b-chain.
Putting these two elements together, I disagree that there is an "objective baseline standard for accuracy". IMO, there is a "subjective baseline standard" for what constitutes an environment conducive to good mastering and typically that means a fairly inaccurate freq response both deliberately and due to unavoidable circumstance. There is no objective baseline standard, all mastering studios are audibly different and all mastering engineers are individuals and at least somewhat different. I've been in some wonderful mastering rooms and also some which I felt were poor enough to preclude me from producing my best work but which don't preclude (and actually aid) other engineers to produce top quality results and, that's even in those cases where my idea of "top quality" is actually identical to another mastering engineer's! What you are "trying to aim for" is based on a fallacy that mastering suites (and mastering engineers) adhere to some objective standard. The actual target you are "trying to aim for" is a creation of your own imagination, of what you think/feel a mastering suite should be, not what they actually are!
I don't dispute your (or anyone else's) right to EQ your equipment however you wish. I also don't dispute that the results of that EQ may very well sound better to you and possibly even to me. What I'm saying is that acoustics, mastering suites and the personal act of mastering itself is a warren of rabbit holes, a complex set of objective and subjective variables. Reducing this to a single variable, reliably/easily treated with a single and rather blunt tool (EQ), is over-simplifying the issue to the point that it's just as likely to be counter-productive. I realise that system tweaking is an integral part of audiophilia for many and therefore any advice against tweaking is an anathema. The question is, what is our goal and if that goal is not tweaking itself, does tweaking get us closer to that goal? In this particular case, if our goal is to create a sound we personally like, then tweak away. If our goal is to try and experience what the artists/engineers intended then the answer is not so simple; tweaking may get us closer or it may take us further away but crucially, we're never going to know unless we're lucky enough to visit the mastering studio where the music was mastered!
