I think that depends on the store and the individual trainers. I haven't used their classes myself, but I have met two very good positive-only trainers who work at one particular store, and the training area for this store is way in the back in a low-traffic area. One good way to find out which trainers know what they're doing and which don't have a clue is to sit in during a class. This goes for all trainers, though, PetsMart or non...
The reason I suggested PetsMart is because they're pretty much everywhere, and I don't know where the OP is from. But if it makes you feel a little better, I can also direct them to the APDT:
http://www.apdt.com/ - Classes will be a little more pricey most likely, but probably higher quality... But these days, you have to be careful even when selecting a trainer from there. >X.x;<