It's not so much the load as it is to the melting snow underneath when it melts during the day underneath the load of snow and then refreezes at night. You don't see it put it does happen from the heat of the sun no matter how cold it is.
This causes ice dams 'under the snow' that will rip apart all of your seals around everything that is on your roof. Including the roof seals it self.
When I was up in Wisconsin I brushed the snow off everyday.
Note: Don't brush over your decals....I did. :(
Come spring I saw what I did. The decals are brittle in the cold. When I brushed the snow off them, even though it was with one of those soft extension brushes for washing RV's.....I broke all the edges.