A good groomer should know just how much to leave and what type of cut to give him. How often you need to groom depends on how often you comb him and what type hair he has.

If your dog has curly poodle-type hair I prefer what they call the sport cut, which is a 1-2" cut all over.
Silky coats that aren't regularly combed tend to matt, so if your dog has that type hair you will definitely want to have it trimmed on a regular basis.

Talk thru your options with the groomer, then be prepared to buy him a little coat for those really cold days. :)

I would rely on the advice of a groomer.

My dogs have short hair, so they don't need haircuts. Even so I don't like to bathe them in the winter, because I don't want them to be wet and cold. Instead of bathing, I make sure I brush them alot. I might use a damp cloth if they have some dirt stuck on them.

My small dog with short hair typically wears a sweater in the winter, or a coat when we go outside.

heres a pic of my dog in one of her winter sweaters: