As Ross notes
unstepping, storing, and re-stepping a mast are expensive tasks at most marinas. Most of the marinas that require you to unstep your mast do so for safety reasons: a boat up on jackstands with its mast/rigging up creates much more windage than a boat stored with a mast down. If the choice is optional, and the area you are in is not known for high winter winds, storing with the mast up is probably preferable (but loosen up the shrouds/stays a bit). It's probably a good idea to unstep your mast in any event every 5 yrs or so and make an inspection of it.