Depends on what i'm playing. If i'm mixing some early 90's stuff with newer stuff 90's tunes end up between 7 & 8 and the otheres between -2 and +2 depending on bpms. Try and keep it under +4 usually though.
I normally start between +2 and +4 depending on the track... it stops you ever needing to mix near 0 or below.
I think there is a point where it just sounds too fast and rushed... the beats need space to breathe! I find with most things after +6 it's a bit excessive unless it's an old tune with a slow BPM to begin with.