Just curious. My 4x5 camera has levels on it, and I find myself glancing at them just about every time I setup. My 8x10 doesn't have any levels (but does have a nice grid on the glass), and honestly I haven't really felt like I'm lacking anything with out them while I'm shooting or after I'm done and looking at the negatives. It's difficult to shoot my 4x5 with out looking at the levels, since they are already there.
So I'm curious, how many of you use levels when shooting landscapes? Do you find that they help you compose? Or confirm your composition? Or just as an aid when you are shooting on un-level ground before you start composing?