Just tighten snugly, DO NOT over tighten, as previously stated.

I recently found my front one to be leaking because I didn't tighten it
enough, and I had to pop the tire off the bead with a clamp and re-tighten
the valve stem.
I'm pretty sure you could tighten it enough to deform the grommet, and that
would be bad.
My leaker seemed OK at first, but I could spin the valve stem all the
way around pretty easily.
Now that it is tightened and leak-free, it does not spin (easily)...
-Mike