Yup, you do not need to deworm if your ares experiences hard freezes and stays frozen during the winter. You should deworm after the first freeze, then again after the first thaw in spring.
Alternately, if you live in a desert area or one that is hot AND dry in the summer (over 85 degrees for extended periods), you do not have to deworm in the summer. When I lived in Southeastern California (deserty area) I did not deworm from June through September.
When planning a rotation, keep in mind that products with the ingrediants fenbendazole and oxibendazole are NOT, I repeat NOT, effective in adult horses. You are wasting your money.