For the first point, I am assuming you mean in a combo and not when making someone block it and it just has to deal with proration. In some combos, it isn't possible to do all the hits and have the later parts hit and since the later parts are more important, you want to cut it short.
Again assuming you are talking about combos, I haven't really noticed a huge difference between cancelling 3C earlier unless you are going to be out of range if you continue hitting them. However, if you are going for knockdown, it matters of course.
Someone else could probably come up with a perfect combo example but I can't since I generally just break my combos into pieces and stitch them together in matches and suck at remembering combos.