I heard you can get them to fit every time by inserting them properly (opening your mouth, pulling the top of your ear upwards, twisting in, and closing your mouth).
Yes, I'm very quite familiar with that procedure
and it certainly helps to open up the ear so you can get deeper insertion. However, getting the "best" possible sound is not just about getting a good seal; AFAIK there is also the shape of your ear canal and getting the tip angled correctly so that it is ideally firing into your canal rather than at the wall of the canal. This is what I meant when I said it was always a game of luck getting it positioned just right; although I got better at it with time, it was a pain and it only felt "perfect" maybe 20% of the time.
There are many types of tips you can try. For me, the only tips that I liked were the foam kind. They were the only ones that I found comfortable enough, and that provided an adequate seal. The trouble is, these wear out quickly. Upon the first use of a new tip, the sound was magnificent, but after several days it would not expand with as much force and the seal would not be as strong. Thus the games began trying to get deeper insertion for a tighter seal and better positioning. So I was spending $20 every 2 months buying more tips. Of course, this might not be an issue for you if you can get a good seal and comfort with the sturdier types of tips.
With customs, you can skip all this nonsense. But if you have to buy new customs in two years, that's a big expense. You really won't know how well universals work for you unless you try. The nice thing about universals is that they retain a better resale value than customs, so if you find the sound lacking, you can easily sell them with less financial loss. First world problems.