There are very few reasons to ever dump fish store water into your tank, a frogfish, puffer fish, or a sponge would be the immediate entities that come to mind. But I think that medicationg your display tanks is really anicillary to the matter, how about quarantineing them before you slap them out for everyone to see and tap the glass on? Quarantine is where all medication should be administered. Fish don't say in the display tanks of stores that quarantine for very long, their obvious health and vigour make them a quick buy.
And if you want to get REALLY anal on all this, shouldn't your newly purchased fish/invert be going into a quarantine tank first? Granted, how many of us REALLY have a quarantine tank... I have a ten I ostensibly call a quarantine, but it just got done holding some goldfish for my wifes tank... the fact that it held copius amounts of copper makes me wary of ever using it again (which I think is justified since the green parasite medication actually stained 'bubbles' under the silicone... there could ostensibly be pockets of copper in the there too... imho, why take a risk? it's a 15 dollar tank, I'll by another.)
I would agree that good habbits on the hobbyists part are numero uno in importance, however, there are newbs out there that will have a hard enough time as it is buying 20 things and slapping it in their brand new tank before it's finished cycling... they don't know to ask... I can see both sides of the argument...
"Maybe in order to understand mankind, we have to look at the word itself: "Mankind". Basically, it's made up of two separate words - "mank" and "ind". What do these words mean ? It's a mystery, and that's why so is mankind." ~ Jack Handey