Thanks very much Moncho!
Do you crop often when using the 50L ?
I try to compose as close to the final framing as possible so I don't have to crop. But, because I'm usually composing and shooting really quickly on the go as my subject is moving and posing, I can't always get things framed as perfectly as if it were a more pre-planned shot. I try to consider expression and lighting first, and that usually determines when I snap the shutter, with framing and sharpness secondary or tertiary. If I have to crop in post I will, but I try hard to avoid it.
Are you focusing manually when you compose the subject outside the center?
Sometimes, particularly if I'm shooting very quickly and don't have time to put a focus point on the model's eye. I installed the precision screen (Eg-S?) in my 6D for that purpose. I'll often shoot 3 shots in a 1 stop bracketing mode and I might move slightly forward or backward with my camera when doing so to increase the likelihood that one of the shots is pretty close to perfect focus.
When do you choose the 135mm over the the 85mm?
Often it's just what's on the camera at the time or just what suits my fancy. But if I had a general rule, it would be to use the 135mm when I have more room and more available light, like outdoor shots, and to pick the 85mm when I"m in closer quarters and/or lower light levels where its shorter focal length and 1+ stop advantage can reduce motion blur.
I consider those two lenses almost interchangeable, but if I had to pic just one it would be the 85mm purely for one practical reason - I can crop the 85 to get 135, but if the 135 is too long or too slow, I'd be out of luck.
when do you use your 70-200?
When I think I'm gonna want the flexibility of the zoom range plus the extra compression of the 200mm focal length. Basically if I'm feeling lazy that day and don't want to be changing lenses. I get a lot of images I like from this zoom but often, when I'm working on them in post, I find myself wishing I'd used one of the primes for their slightly better bokeh and gentler skin rendering.
And do you think your 135 could help out if you had an f4?
Only if sharpness of the subject is more important than bokeh because you usually increase the former and deteriorate the latter by stopping down. I just usually prefer the compromises of shooting wide open.
Good luck with your search. IMHO, a photographer's sensibilities are WAY more important than slight differences between lenses.