This is a hard one, because the quality of each pet store, even if it is a chain, varies greatly. There is one Petsmart that I will shop at, but the other two I don't even go in the door, because they take such poor care of their fish I can't stand to look at the suffering. The local PetCo is pretty good. The guy has some knowledge and freely admits when he doesn't know the answer and even referred me to visit a forum with one of the questions I had. The two purely fish only stores are OK, but not great. Their fish look OK, but they are not very helpful and their staff varies widely in their knowledge. One of them, the young guys that work there are only interested in salt water fish and won't even ask you if you need help if you are looking at freshwater. I visited two other small pet stores and one I found had good looking tanks, but a poor selection of fish, the other was disgusting so I immediately left. So, my suggestion is find the best you can and work with them. I got my fish at four different stores and out of seven fish bought only one died.
Even on forums you get varying information and sometimes it is wrong, like the forum I read that said that corys like to be alone. Found out from the local lfs that they like to be in groups. I only had room, size wise, for one more fish, but when I finally got my cory a friend (out of them for a couple weeks) he was overjoyed and they have been best buddies since the first moment I put the second one in the tank.
So, the best advice I have is to learn as much as you can about fish care, to get more than one source from information presented on a forum and try to help educate your local lfs if they seem to be less knowleable than you. I've told several about fishless cycling, so at least they have the knowledge even if they choose to ignore it so that they sell more fish.