It's never a recommendation or good practice to do that. Even in cold climates where I am, simply insulating the vent is all that's needed. If the issue is ice collection at the exterior discharge point, then that's a discharge configuration design problem. Why go to the bother of using high effiency, energy saving appliances and then go and add more energy consuming things back into the system afterwards?
I agree with GMcD. You can insulate the vent and that will be sufficient. If you have a long run of the vent in the unheated space... then that is an installation issue that needs to be addressed.
