If saltwater, I drip acclimate.
If freshwater, especially when bought online, I find out the pH and general water conditions the fish come to me from. The pH is almost always lower than what I have, so I usually throw in some new driftwood prior to dumping the fish out. Usually overnight shipping does not lead to issues, but I find it better to get the fish into fresh water rather than float and wait; the temperatures are usually pretty close regardless. Now, if you're talking fish like discus or wild-caught plecos or cories, I might drip, but it depends on the water in the shipping bag.