Mast up
In our area, Montreal in Canada, where we have a lot of stroms and snow in the winter time, 99% of the boats are stored with the mast up. Those that lower the mast are, most of the time doing it to work on the mast or hardware. The vast majority leave the boats without any type of cover. There are around 400 boats at our marina. When the snow melt, it takes all the dust with it and the boats are fairly clean for the spring launch. Boats here are on the ground from mid October to mid May.I do not think that leaving the mast up is a problem at all.