I always measure the height of the wall. If the wallpaper is too tall, cut it off at the bottom edge. It will be less noticeable. You are probably going to put up baseboard molding or carpet that will help to cover an uneven edge.
If the room has odd shaped walls and you haven't put the dollhouse together yet, make a template or pattern of the wall. This is great for attic rooms that slant, especially. When I built REAL GOOD TOYS light house kit, I made templates of each wall and each floor. The top of the walls are smaller in width than the bottom of the same wall. This was a life saver, for sure.
I make the pattern or template the exact size of the wall. When I trace the pattern for the BACK wall onto the backside of the wall paper, I extend each side about 1/2". I wall paper the BACK wall first, wrapping the extra wall paper onto the side walls. Then I wallpaper the side walls. I push the wallpaper back so it butts up against the back wall, covering the extra wall paper that was wrapped around. This make a nice finish so there are not any gaps in those back corners. I like to use Grandmother Stover's glue. But I take it out of the jar, and put it into a glass jar a bit bigger with a teaspoon of water. Then I put it into the microwave and heat it until I can stir it with a craft stick. I might have to add more water. I want it the consistency of a thick maple syrup. I use a stencil brush to them apply the glue to the back of the wall paper. This is a little trick I learned from taking a Brooke Tucker room box class, back in the day.