I'm from Idaho where "steep" is a very relative term. We have a section on our farm that is 30 degrees, more or less, about what you describe. The horses go up and down it all the time--we often put their hay at the bottom of it to make them work a bit. And, yes, I've ridden Mr. Big up and down it numerous times.
That being said, when it gets that steep Big likes to quarter both up and down--big zigs and zags so he isn't fighting gravity quite so much. I generally let him.
However, down at Hell's Gate State Park there are numerous trails that are steeper than anything we have on our land. We have to stay on the trails. We take it slow and Big has never had a problem. I often have to force him to stay slow--he sometimes wants to trot on the steep stuff, sometimes even canter going up. I try to keep him at a walk both for my comfort and his safety (which means my safety, too!).
Take it easy to start and get your horse in shape. Think about it: if you were always on flat land how would YOU like to carry a load up a series of steep hills without having a chance to get in shape first? Horses have muscles, too, and they respond to training just like ours.
"Don't just feed it! Ride it!" And the People said "AMEN!"