In all honesty, if one does not listen to a lot of female vocalists, then maybe to choose not to EQ is an option. However, if one does listen to a lot of female vocalists, then I believe (IMO) it is a must to EQ with the HE-400s due to the sibilants. If one simply refuses to IQ, but loves the voices of the female singers, then the HE-500 is a great option, but does cost $300 more.
This simply is one item that drove me nuts with the HE-400. I guess if I had purged all female artists from my music collection, then the HE-400 (or, all 3 pair) would have been a good fit for me. But, I will say, the HE-400 is one heck of a "teaser" headphone for the price.
Even male vocals have sibilants you know. So im my opinion, if you listen to vocals at all then EQ is a must. But again, not all tracks with vocals have sibilants, it's down to the recording.