I'm not sure why everyone assumes an A-frame should be decorated either "rustic" or "country." I suppose because the majority of A-frames are in snowy mountain landscapes, not suburban neighborhoods. Still, does every mountain lodge have to look the same? I'm not a fan of either style so I won't be decorating in those fashions. I understand the A-frame presents serious challenges and restrictions. I won't be adding Louis XIV chairs and armoires anytime. As the house itself is a bit of art in itself, I think it is best to keep the surrounding accessories pretty simple. I have spent hours Googling "A-frame remodel" and "A-frame decorating" and there are little to no resources available on the subject.
I've even come across someone advising an A-frame owner to hang artwork on the angled walls. I seriously advise against this (and frankly, don't understand it--would the top of the art attach at the wall and the bottom hang about 2 feet from the wall? Would you actually attach the art at an angle?) Enjoy the walls for what they are because the tongue and groove can actually be very soothing and spa-like. It also casts a very warm glow.
From decorating my own house I find the styles that seem to work best with an A-frame are modern, transitional, craftsman and mission.