Watching any possible purchases while they are at the LFS is always a good thing. Gives you the oppertunity to watch that they are eating (something some breeds don't take to well in captivity), as well as looking for signs of stress and/or desease.
Unfortunately, any fish you happen to see at your LFS that you like, may not be the exact same fish you see there when you come back a day later, let a lone a couple weeks later. Even the bestest of LFS owners & sales reps for those stores, are in the business to SELL their stock. They also are continually receiving new stock, which may or may not be housed in the same tank as "your" potential fish is.
That also brings up another possible problem... a healthy fish becoming contaminated by a new addition to that tank. A new fish "MAY" look perfectly healthy to our eyes... but you don't have any idea what it may or may not have. And, once its put into the same tank as any other fish, those original fish now may come in contact with something that will harm them.
Okay... I guess summing all of that up... anytime I add a fish, I just plan on doing my own QT, with preventive treatments... knowing the risks and possible problems. I highly recommend you research into this as a plan for yourself also.