[Joe Johnson]

Worm gears are notoriously inefficient. That is why they don't backdrive actually. They throw away 60-90% of the power that you put into them (said another way only 10-40% of the mechancal power you put into the worm gear shaft comes out the other side as mechanical power on the worm wheel shaft).

The reason you care is that if you are trying to do a fixed energy task (e.g. lift a 140lbs robot 2 feet in the presence of a 32.2fps^2 gravity field) then your power output is going to tell you how long it takes to complete that task. If only 10% of your input power ends up going to that desired task (i.e. lifting your robot) then you need either put in 10X the power to finish the job in the same time or you need to take 10X as long if you put in the same amount of power (or some combination 5X the power lifts in 2X the time, 3X the power lifts in 3.3X the time, ...)

Bottom line, if you can manage age, I believe it is preferable to have a lock or a brake or an over center linkage or 10 other ideas your team could come up with that will keep your robot from back driving that are not a worm and wormgear pair.

YMMV.

Dr. Joe J.