Agree that it depends. My instructor started me and Huey with cantering with that "sitting the trot" and *entering* the corner (rather than exiting it like the others said). But his trot is gigantic and he's prone to star-gazing which makes his back hollow - and that isn't helped by having someone attempt to sit his trot (not just me - even my trainer has a hard time with sitting his trot) - and that meant that I was having to try to get my leg into position for the request WHILE bouncing, and it meant that every time we got into the canter, I had to spend the first several strides regaining my seat from the sitting trot transition.
I did some experimenting on my own one day, and discovered that it is MUCH easier for both of us if I ask him to canter from a walk. When I do, I just ask him for some extra movement in his walk, I do a half-halt, and then I reach back with my outside leg and just touch him behind the girth, and he picks the canter up right away, smooth as glass. Or, that's how it is 90% of the time. :P The other 10% of the time, he busts out into this rocket-speed trot instead, and I have to bring him back to the walk, give him a kick behind the girth with that outside leg, and then ask for the canter again. Never have to do this twice.
I know at some point I'm going to have to learn to sit his trot, but thinking that I couldn't canter without sitting that super-size trampoline trot held us back for months.