I think, like rootboy said, getting the expander below the threads is the generally accepted principle to follow.

I can see the need to cut a Technomic if you have a small frame, need the height of the stem, but not all the available height. Slamming the stem all the way down could result in putting the expander in the butted portion of the steerer, which isn't a good idea. The obvious solution would be to shorten the stem.

Hands Up (Or Down)! Adjusting Handlebar Stem Height on Your Bicycle