When do I freeze tamales?
I would steam them, let them cool, then freeze them.  If you buy them in large quantites from a vendor or from a fund raiser or something, they come already steamed.

As for prep, I would think the microwave is the best answer, as that ought to keep them nice and moist while they are cooking.  Steaming them again would probably work, but might take a while.  And while I haven't tried it, I suspect an oven might dry them out too much.