I've always used a sheetrock saw too and never had any problems with the rough edge. I just cut and go. It gets covered anyway.
If your using remodeling cans (depending on the brand) a tight hole sometimes can blowout the drywall when you shift the angle as you "jam" it up. The key to cutting that hole is: not too big, not too small, but just right........
I do use a 6-5/8" greenlee hole saw too, but more for 2X2 ceiling tiles. At $40 for the cup (no arbor) you'd probably be better off with the rotozip.
