The first application of polyurethane finally dried on my red oak
baseboard. I sanded the baseboard and used a tacky cloth to remove the
dust. Should I wipe down the baseboard with a rag dipped in mineral
spirits before I apply the next coat of polyurethane?

Why would you want to do that?

The mineral spirits will remove any residue left behind by the tack cloth.
Nothing wrong with doing this if the poly has cured sufficiently.

I certainly never have.

Therefore it's not rational?