Every takeoff should be the same therefore the landing gear should always be raised if if only a circuit is planned. Leaving the gear down is negative training as it doesn't reinforce the gear down as part of the prelanding check. You are inviting a gear up landing for a new pilot in this situation. As for ab initio training I don't see any reason why not as it is flies the same as a regular C 172. The stude will need a little extra training for the VP prop and retract gear but that is not a big deal.