Unless you plan on snow camping, a +20 bag ought to do you fine. Even a +25, if it is honestly rated, and if you have some extra clothes to wear inside it on extra-cold nights.

However, this suggestion comes with the usual warning that different people sleep warmer or colder, they sleep on different pads, and in different shelters, and all of these factors can make a difference. (Especially the pad that insulates underneath you!)