Do I need to strip my cabinets before painting them or can I just scuff them up with sandpaper?

Chemical strip is easier than sanding, but messier and toxic. Your call. A light sanding; scuffing, might not leave a smooth surface to show off new paint.

I would scuff it and paint with enamal paint it dries hard and stands up best to wear, tear and cleaning. If you put a couple of coasts light sanding in between coats with XXX steel wool and it will be smooth as sprayed

I guess I must be just lazy, but I have done much refinishing and painting in my long life. If you sand off the dirt and the gloss of the present finish, you should be able to apply new paint without any trouble (as long as the original paint is not flaking, alligatoring, or chipping off). Anyway that's the way I've always done it. The purpose of sanding off the old is simply because the new paint won't stick to the old if it hasn't been sanded. It needs a rough surface to grab. But I would never suggest that you strip off the old paint. It's a HUGE undertaking and just not necessary for repainting. I agree with gooch that you should use enamel paint. But use the Latex enamel instead of the oil base. It's much easier to clean up and works as well as oil-based. I suppose you should lightly sand or steel wool the surface if you use a high gloss enamel and want to give it a second coat. But, to be honest, I never go to that trouble. And I haven't had a problem with just putting the second coat over the first.

A little tip...I would 'test' paint the inside of the cabinets' door before doing the outside of cabinets door.