Personally, I think you should be careful wherever you purchase your fish, but some stores are better than others. And it also depends on when you go. Sometimes they get bad shipments. I purchased otos there for a dollar ea and they actually surprised me as to how hardy they are. BUT, shortly afterward and still to this day, I've been fighting a VERY resistant form of Ich. I don't know if it hitched a ride on the otos or not, but they were the last fish I purchased.
My daughter got a betta from Walmart for her birthday and it turned out to be a very beautiful and healthy fish, go figure. Shortly thereafter, she bought one from Petsmart and it died of ich.
I'm not saying you are doomed to get bad fish at Petsmart, all I'm saying is to be careful and visit the store a few times during the week before purchasing, just to be sure they aren't sick or in a tank with sick fish. That's all.
The only place I would NEVER buy a fish is Petco--EVERY time I to there, almost every tank has either a dead fish or many tanks with quarantine signs. At least the one by me doesn't seem to take care of their fish at all.