I have a triple crank and almost always use the center chainring. I'll only really switch the chainring if i need a higher or lower gear. Since I also ride on campus, if I'm in with pedestrians (sometimes I have no choice) I'll shift into the smallest chainring to keep me under 6 mph or so...

So to answer your question, I almost never cross the chain and never do the big/big or little/little combo. I'd estimate that most of my riding is done in one of 4 or 5 gears though hahaha