The subject of double-cutting on the wall always gets a rise out of paperhangers. It seems that some installers shy away from it because they've seen what can happen when a double-cut job is botched. A botched double-cut job occurs when someone uses too much force to cut through two sheets of wallpaper, and in the process, cuts through the paint as well as the sheetrock facing.
Some hangers trim their material on the table, some on the wall.
I've done both and personally have opted for cutting on the wall most of time. If you are new to double-cutting, or want to give it a try, it can make your job look better in many instances, and particularly with the inlaying of borders. Just for now, put away the box-cutter, Olfa knife, or any other cutting implement with a handle. Later on, when you get a feel for the force necessary to cut through two sheets, you can go back to a handled instrument. Practice Session Use three pasted strips of wallpaper.