When I first started my tank I needed 6 tiger barbs and 6 cherry barbs. Those I left up to the LFS and I wish for the cherries I had selected individually. After that I was only purchasing 1 or 2 at a time (Oto's, cory's, BN pleco, dwarf gourami), and I was specific with the one I wanted.
It's nice at my LFS since the tanks are normally not very crowded, so its easier to get the one I want when there are only 10-15 in a 10 gallon tank. I look at it like buying a car/house/etc. Your in for the long haul, so I expect the pick of the litter.
I just recently purchased a dwarf gourami where there were about 15 or so in the tank (10 gallon). I was asked 3 times by 2 different employees if I could be helped. I politely said I will be getting one of these I just haven't decided which one yet. They left me be, and I was able to identify the alpha male of the group (all males) whom was the most aggressive and healthy IMO. I am hoping that when introduced into my main tank (2-3week QT until I'm sure he's healthy) due to his semi-aggressiveness and good health that he can take care of himself with my tigers.