There is only one lfs here in Portland that I'd allow their tank water to mix with mine, as they're more than a lfs, they're also a breeder and world-wide distributor of cichlids and gouramis and they do know their water. If you live in Portland, Oregon, go to The Wet Spot. A really great place for healthy, disease-free fish of all kinds and helpful advice...they've helped me a lot. Their breeding facilities are a few blocks away from there. If your favorite species is cichlids, go there!
Otherwise if from some place like Petsmart, I float the bag for ten minutes, then allow it to fill with about 30% of my tank water, let it set for another 10, then net the fish out. I once tested the water from Petsmart and the results were fine, actually. Once I get my 55 gallon set up downstairs I'll be practicing the art of quarantining them in a 10 gallon for a week first. Probably overkill, but better safe than sorry I suppose.