To avoid that problem do as suggested: check for burrs near valve hole; tighen but do not over tighten the valve stem retainer.
Another suggestion: grab an old inner tube and cut about a 1 square inch patch out of it. Cut a small hole dead center of that patch and slip it over the valve stem before putting valve stem through the hole in rim. That little extra padding wil prevent tube from get pinched/cut.
Rudy and Kay/zonatandem