When you top your plant, it takes time (2 sometimes 3 weeks) for a topped plant to start growing again. Instead of 1 top, you'll now have 2, but with a couple week delay for that process to start growing again. If time is NOT a constraint, go ahead and top. IMHO, LST (low stress training) is a very good way to keep the grow at a certain height, and you can pick that height. Nothing is taken away, and growth keeps on growing. It is explained in the grow FAQ on how to do this. I recommend it highly.
Nim