I keep it at flat, otherwise I get very bad distortion, especially on low frequencies. It's happened with any soundcard I've tried it on, and at least 4 different sets of speakers.
My NewQ Platinum does all the equalizer functions for me.

The only software-controlled equalizer I ever used that worked great was the one that the Aureal Vortex cards used. I could increase the bass on it, and lower the high frequencies - very loud bass, without distortion. I can't do that in Winamp, or with the equalizer on my Santa Cruz card - always gives distortion if I increase the levels above their default flat level.