I was just at our vet's office yesterday because
yikes
Trixie had two fleas on her!! We got AdvantixII flea preventative (because I didn't want to give it to her since she is usually only in my yard
) and asked about stopping it during the winter......
Our vet said that it has been a horrible flea season with even indoor only pets getting fleas and to call in December to see if the area is still having a flea problem.
So, I would call your vet's office and see how bad the fleas are in your area.
As far as the heartworm preventative is concerned.....I would give it year round.