I honestly think leaving the bag to try to match temp is often a dangerous game. I'd rather the fish get some oxygen (ie: get that bag open). Again, have never had a fish not cope with water changes and that can often change the tank temperature. I think if you are going to lose a fish because you didnt float it, you maybe were going to lose it anyway. The slow drip method should be something you can start immediately without floating. River and lake temps change a lot when it rains, remember... Infact if you want Cory cats to spawn, guess what you do?