Oh yeah they definitely check at skytrain stations, but I rarely, if ever, see them on B-Lines. Maybe i've just been lucky! Even though I always have my upass on me, I still get nervous around those guys.
Just thought I'd mention that yesterday on the 99 B-Line cops were checking tickets. Maybe they're trying to crack down on fare evaders because this is the first time in years I've had this happen on a bus. So yeah, pay your fares!