So by EQing, you are trading soundstage coherency for frequency balance. One the primary parameters that determines audiophile sound quality is phase, often called "group delay" in specs. It's what can make one headphone have great soundstage and another one collapse everything into left-right-center.
The short version is that by EQing, you are mangling your soundstage in the pursuit of a flat frequency response. Genre of music is relevant though. If the music you listen to is already massively EQed by the recording engineer, a little more ain't gonna hurt. If you are listening to something that was recorded well though, you're much better off buying headphones that already have the frequency response you want and not damaging the signal any more than necessary.
This has not been my experience. But I also usually use a crossfeed plugin which vastly changes soundstage as well (for the better). So it could be that I'm fixing any phase issues already.
I use some of my headphones for mastering, and need them to match my mastering monitors. Using EQ to get their responses closer together (there are none that would match precisely) is a very useful and workable solution.
It is just a bonus that it makes them even better for casual listening.