How are you planning to finish the cabinets? If you're going to use new stain and a transparent finish, then you need to get the old finish off, in my opinion. There are finish removers that do that task, but I use them in conjunction with sanding.
If you're re coating the cabinets with an opaque paint, then they need to be clean. Use TSP to get all the grime and grease off and then sand enough to provide a "tooth" for the paint.
Is the wallpaper sound? If so, I'd just paint over it. But if it's pealing off or bubbling, then I'd remove it all before painting. I've never removed wallpaper, but I imagine there are numerous utube videos on how to do it.
- Jack