I like no-restraint torture sessions -- they save time for the torture! Plus it's really fun to play a game of "don't move" with someone I'm hitting in the balls.

That said, I don't think it necessarily indicates that the slave is "more submissive." The squirming can be involuntary, especially with a sub playing very near the edge of his limits. With a squirmy sub I do prefer restraints since he can get in my way otherwise.