Tiff does not produce higher resolution than either jpeg or raw. What it does, in contrast to jpeg, is to deliver an uncompressed image. If shooting at the highest quality jpeg settings, you should see no deterioration of the image. However, if you do edit it, you should always resave it as a tif. Never save a jpeg as a jpeg. That is where the problems start coming in.
I suspect the 20D offers jpeg rather than tif in the interest of speed. It maintains its highest burst rates when shooting in the jpeg format.
I'm interested in purchasing a Canon DSLR. I was initially interested in the 20D but noticed it didn't have a TIFF setting (only jpeg and RAW). I thought TIFF was important to have as it potentially provides the best resolution. I don't always want to have to post-edit my shots (in the case of RAW). Any comments on this?