Good question, Tom -- I've never done any root pruning when I was transplanting, other than if the roots have gotten potbound by then I'll cut through them (in a top to bottom direction) with a knife before setting in the ground.
What I was actually originally thinking of in terms of "root pruning" is that I've read that tomatoes will produce ripe fruit sooner if you cut into the roots with a shovel once the plant is grown. I guess I need to do some experimenting!
