It depends on what you're talking about. There are some people who simply don't canter well and bounce around. (bounce up and down instead of moving with the canter) :) There are certain disciplines (like luvs2jump said) that jump more off the half seat. Hopefully the rider's seat is quiet, but I think it's easy to get into the bad habit of not being still. (I've heard of it refered to as the "hunter hula" or even "butt circles")
But there are people who actually post at the canter. I think it's strange and uncomfortable but I know a few trainers who teach their young kids to post at the canter (supposedly makes cantering easier for them) or post the canter themselves (supposed to help a horse regulate his pace, just like posting at the trot can).
A hand gallop is nothing more then a fast canter (the challenge is doing it well). Sometimes they'll ask for it in an (H/J) equitation flat class, or sometimes in a handy hunter, classic, derby, or eq round they'll ask you to hand gallop a fence. I was at some breed show (Arab nationals?) and in some flat class they asked for it. And I think foxhunters/eventers hand gallop during the long stretches.
As you get more advanced in your jumping you'll be able to eye a fence and be able to tell how many strides out you are. Comes with time, practice, and riding the approach well.
Last edited by upnover; 10-14-2010 at 11:35 PM.