I read about this all the time, but have never personally experienced any film flatness issues w/ 120 cameras. It won't happen if you shoot a red window folder because you get the opportunity to snug the film tight on each frame. If it were going to happen I suspect it would be w/ a TLR, where the film makes a sharp bend before laying in front of the film gate. I live in hot weather climates, and when it's 90+ degrees outside, I suspect that no film emulsion ever made is going to retain a kink in that weather for very long. In cold climates, I could see it theoretically happening. The choice then would be to either burn a frame, or shoot it and see what happens.
Never experienced focus issues or anything like that BUT I did one time process 120 that had been sitting in my c330 for months - after firing off a few BS frames at the end to finish the roll.
While loading the film (Hewes) this particular roll was pretty much impossible to spool at some point - as it kept binding and spoiling the rest of the load. I finally had to just man handle that portion of the film to get it to play nicely (hands sweating - all of that). After processing guess which frames were the most trashed? The ones right around where I shot the first finish-it frames. Amazingly the rest of the film was alright but what I took away from it is that if it's been in the camera to let it sit in finished rolled on the spool form for atleast some time before processing as that portion of the film had definitely taken up a curve as a result of sitting.
Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.
I processed a 120 Rollei 400IR last night which had a definite kink in it. It had been in a Bronica SQ back for approximately a week. I shot one frame and put it aside. I finished the roll in a 2 hour period at the end of the week. The kink was between frames. I will avoid doing this in future by loading just before use, and finishing the roll in one session. Alex