IMHO, when it comes to the 70-200 L IS glass, you are being penny wise and pound foolish if you do not get the Canon TC! The 70-200 2.8 L IS isa fabulous lens, and the Canon TC seems perfectly matched to it. There is virtually no difference in performance and IQ with or without.
And while my Sigma TC works well with my Bigma, it just does not seem to be quite as sharp on my Canon lenses. Anyway, that's my 2¢ worth.