The granules on the shingles will give you good traction provided all you have is wind and rain. Once it snows, however, the grandules get covered with snow and crawling up onto a snow covered roof can be very dangerous.

Best to talk to some roofing contractors in your area. In some areas of the USA, "winter" is when you have to put on a sweater or even wear a rain coat, and you can do that kind of work year-round.