I'm constantly trying to improve my composition/framing to get more dynamic and better shots, and 99% of the time I've of course been relying on my view in the finder for framing. Sometimes you can't get the kid AND the sand castle AND the water (or whatever features you want framed) in the shot without contorting yourself into an odd position, making it hard to get the angle right. Just a few times too often I've fallen into the trap of leaving too little room at the edges only to find that angle correction is needed and part of my subject is thereby cut off, ruining the image. I'm thus learning to leave at least A LITTLE cropping room, which the 5D3's 22MP accommodates easily.
On my 60D because of the few/far between AF points (and the practical usability of, pretty much, only the center point) I often had to crop just to get a well-composed image. It worked fine in general.