Use a speed that you would normally use for a downwind to base & base to final. Somewhere in the ballpark of 65-75kts with about 1500RPM should work. Try it with 0-10-20 flaps too. I wouldn't do it over 65 with 20 flaps though. The whole idea is to find out how hard you can pull before something happens so try it in different configurations, power settings, bank angles and speeds.
It's gonna be hard to snap a 172 on its back. If you have some power in a single and do an accelerated stall and to the left and hold it in for a bit the gyroscopic precession will actually help roll the airplane back to wings level for you. It does the opposite if you go the the right....
Just make sure that you and the student (obviously not at the same time) are pulling strait back on the elevator control. With a yoke, its really easy to mix in right aileron when pulling hard with your left. With accelerated stalls, remember that because of the higher airspeed the rudder will be more effective.