If your 70-200 is the f/2.8 version then I would say that using a lens collar is needed. If you're using the f/4 IS L version, I believe that you might use it without the lens collar but only occasionally, the Rebel XTi is very light.
Although I have a 400D (Rebel XTi) and a 70-200 f/4 IS, I've not yet used them both together.
When french mag Chasseur d'image tested the new 70-200 f/4 IS (on a larger and heavier camera than the 400D) then came to the conclusion that one could use the lens without the lens collar but if a 1.4x extender was added, the lens collar would become almost mandatory to get good image quality.
A lens collar also make switching from portrait to landscape composition very fast and convenient.