It depends.
Our national federation allows only 36 rated tires during its events.
This means that we have to use 36 tires during summer and winter.
Sorex 36 tires do not seem to work well in the winter while you can get a decent amount of grip with 36R sweep even in the winter.
Of course this a special case and normally one would never use 36r tires when it is 5degrees C outside but I thought I should share my experience.