If you're just EQing to fit your ears, like adjusting minute upper mids energy, treble peaks etc it doesn't really affect performance, maybe the soundstage suffers a bit but that's it. I've found that, however, playing with the bass REALLY impacts performance, adding just a few dBs will drastically change the details and characteristics of the mids. Usually I EQ the bass only when playing at very low volumes, and turn it off when listening during daytime at normal volumes