Only extremely shit/cheap clubs would have a mono system. Although a ton of quality is lost in most clubs due to poor equipment, badly worked out acoustic placement of speakers, and general club usage they will still have a stereo capable system.
The only parts that really need to be mono are low bass lines, kick drums (actually, anything that is mostly lower end bass should be in mono) and most vocals (unless your doing some kind of effect on the vocal or vocal effect
Two reasons iirc, the human ear has trouble working out distances of bass when its in stereo so it can sound wierd in a club. Also if your pressing to vinyl stereo bass can make the needle jump about on the record which of course is not good
ten