@Latitude rides on A/S Marathon GTs during the winter and I believe, at one point, he decided to park his bike. I can't comment on what conditions others are riding on during the winter but I would certainly err on the side of caution and install A/S tires as a minimum requirement. Sometimes it's hard to avoid ice especially if its there is snow covering it. Even tires with about half the number of studs as most other winter tires such as the Schwalbe Winter K-Guard would provide peace of mind when encountering small patches of ice and won’t break the bank either.