how and whether we acclimate depends on how long our new acquisition has been bagged.
if it has been bagged for over an hour, we simply match temp (floating the closed bag for 20-30 mins) and add the critter to the QT.
if it's under an hour, we use the drip or dip method for 1-2 hrs, depending on the critter.
the reason is this: ammonia (NH3) builds up in the bag water, however, the pH drops due to respiration and waste production. at these lower pH levels, the accumulated NH3 is relatively non-toxic, however, when the bag is opened, the fresh supply of O2 causes the pH in the bag water to rise sharply, thus causing the NH3 in the water to reach toxic levels, which in turn burns the gills of your fish.
that being said, lots of folks use the drip or dip methods, so you kinda need to pick a method for now and go with it.
just to illustrate this, here's a test for NH3 for shipping bag water vs. tank water:
