Since you are asking about a tree, I assume you mean a single specimen and not a hedge. I suspect the answer that you would get from many people here is "Never": Why Topping Hurts Trees from the "Plants Are Good" site by the International Society of Arboriculturists. I spoke with one of the horticulturists here about it as well - he suggested that the best long-term solution is to remove the tree (if there's a reason to top it, e.g., powerlines) and plant something more appropriate to the situation. I realize that neither of these are the answer to the question you asked, though.
I'd agree, a cedar should never be topped. It'll ruin the appearance. Just let them grow naturally Like this . . http://www.pinetum.org/PhotoMPF/cedlib2.gif
Michael, I suspect the cedar in question is a Thuja, perhaps even Thuja occidentalis given Don's location. Common names strike again!
