Should you sandpaper a wall before painting?

Personally I would strip the paper but,

Yes, providing the walls are in good shape the paper has no overlap splices.
Patch holes with patching plaster and use sanding block till smooth/ flush. If there is loose paper, use wall paper paste to secure. Last cut it, don't pull. It could wind up in a bigger area to repair.

Important thing about patching plaster, good for small nail holes and minor stuff. Bigger holes need Drywall wall compound. AND! use drop cloths always and cover/ block off doors and openings. Cold air returns and heater vents or you will have dust everywhere.

One or two small holes hold vacuum next to area as you sand

I would use a Sherwin Williams Colored oil base primer first which will aid in using less paint. Color would be same as what you are painting. Then apply your Latex paint Preferably Sherwin Williams over top.