It certainly does vary by store, but in general, retail stores hire retail salespeople, often without any experience in dealing with pets/fish.
In the very uncommon event that they get knowledgeable employees that they allow the time to properly maintain the pet/fish section, then yes, they can be good places to purchase healthy fish at a great price.
Around here, both Walmart and Petco have more dead fish in their tanks than live ones. Every time I go in, I alert the employees or management to the dead fish in the tanks. Not once have I seen them do anything about it, even seeing the same dead fish in the tanks, days later. As a result, I absolutely refuse to buy fish from either of them (in this area). And the local Petsmart, while having healthy looking fish, there's a 50/50 chance of introducing a parasite to your tank when you bring one home. I still buy there on rare occasion, but I always quarantine anything I get there. Still, I often end up having to spend more to treat the ones that do have health issues, if the issues only become visible after the 2 week return policy. I find it much safer and more reliable to just wait until the same species are stocked at the LFS, or purchase for just a little bit more money at the LFS in the first place.