If the water parameters of the bag water and the tank water are the same then there's no need to acclimate. If they are not then it's best to slowly change the bag water into tank water so that the fish doesn't stress over the sudden change.
You can have problems with the drip method when a number of factors come into play. Ammonia in the bag, bag ph is low, and the drip (tank) water ph is high. Ammonia becomes extremely toxic at higher ph but is virtually harmless at lower ph levels. So in the case where there is ammonia in the bag, dripping the higher ph tank water into the bag can make the ammonia suddenly and fatally toxic to your fish.
A way around this is to use a good declorinator while doing the drip, such as Prime. What I also do is to dump the fish and bag water into a small bucket. Then I add a few cups of tank water to the bucket and immediately remove a few cups of water from the bucket. I repeat this every five minutes or so until the bucket water has become tank water. That way as I'm slowly acclimating the fish to their new water parameters I'm also removing any harmful ammonia.