No Mustering in Winter (9.21 specifically disallows it).
At issue was the conflict between "no Force Marching" for Musters and, "auto-Forced Marching" in Winter, so in the interests of simplicity, it was disallowed.
Since you can Homing Move or Quartering Move units into shelter (for free) on an individual basis, disallowing (paid) Musters was judged an acceptable simplification. The winter rules are already pretty complicated.
The main effect was to make it a little tougher on Athens in Sicily, which is IMO good from a historical and game perspective.
If wish to use a house rule allowing Winter Musters, either with or without auto-Forced March, it would probably have a very minor effect.
