In my experience with my breeding rats I've learned to never tail them.. Every time I clean the tubs I scoop each rat out and put it into the large holding tub without ever grabbing its tail. They seem more likely to show aggression if they are fearful. They learn that it is a fearful experience on cleaning day and they are held by their tails which is uncomfortable and scary.

I also try to handle every baby once in awhile so they don't fear me. It seems to work pretty good. I haven't had an aggressive rat in my colonies in about a year or more. The few that were, which were always Siamese for some reason, were fed off or put down to be frozen for later.