I saw a battery operated product for trimming wallpaper at some trade fair a while ago. it was called eazytrim or easytrimmer
the tool was pressed into an angle or corner onto the wallpaper and it had a sewing machine type action which perforated the paper and left a very neat, clean edge.
amazon have the details but it's discontinued
got a /lot/ of wallpapering coming up and would like to make it as easy and as neat as possible. I've got a rotary wallpaper cutter but I find it to be hit and miss at best.