There's a school of thought about just dumping the fish into a large net as soon as you get it home and dropping it right in the tank water. I have had several fish that have been shipped to me, or shipped to someone at a fish club event then given to me, and I think its far more dangerous to float them and let that ammonia ridden bag water heat up. If the temperature difference between the bag and your tank isn't extreme it shouldn't be a massive difference. Unless its a species that requires very high or very low ph, the chances are the water it comes from and goes into is probably not different enough to warrant keeping the fish in nasty water any longer than necessary.
Again, common sense goes a long way here, if you have a very low ph blackwater setup and your dumping a LFS fish in, it you probably don't want to go from 7.6 ph to something in the low 6's. I haven't acclimated a fish by floating it in quite some time except for one I got this past winter who's bag got way to cold. In that case I did cut the bag and slowly introduce some of my tank water while letting him warm up a bit. I always opt to place my large net over a 5 gallon bucket and dump the whole contents of the bag gently as possible into the net, let it drip for about a second and then dump the fish. Some caveats for me are I recently received a shipment from the west coast, I dumped the fish into the net then dipped the net for a few seconds in another bucket of tank water before introducing the fish to the tank. Again, I didn't leave a fish who spent all night in a bag floating around the top of my aquarium.
