The big systems should be eq'd. Although around here it seems when a rig is used for an electronic event, it's often not sent with one of the company's top engineers, so it's not always eq'd right, if at all. But even if a rig is set up properly, if the event is primarily vinyl, when someone switches to cd's or laptops there is a noticeable sonic difference between mediums.
Regarding soundchecks, even if you don't have one scheduled I've found that if you can get to the venue when sound is being set up, most of the time the audio engineer is happy to sound check you, and if he's good it gives him (or her!) a chance to rinse out the system. If you're playing a club on a fixed system you usually don't get that luxury though.