Ideally they reach zero velocity the instant the legs touch the ground. There was only one hard landing (that didn't result in a fireball) so far, and the legs are designed to take the remaining speed (they are replaced anyway and not reused).
They also have no other option. Even one engine at lowest throttle (70 %) is powerful enough to lift the almost empty booster again, so they cannot hover (which would make things a bit easier, at the expense of needing more fuel).
I can't find the reference, but I remember reading that the landing is complicated by the fact that even at minimum thrust, with the tanks nearly empty, the ship is so light that even with a single engine, it would accelerate back up after hitting zero velocity. So if the engines are restarted too early etc. it could "miss" the ground!
(If someone can confirm/deny this? I'm seeing approx 28T dry mass, thrust per engine of 66T and min thrust of 70%, but that dated guess work from http://space.stackexchange.com/q/4466/ )
They also have no other option. Even one engine at lowest throttle (70 %) is powerful enough to lift the almost empty booster again, so they cannot hover (which would make things a bit easier, at the expense of needing more fuel).