There may not be any "rule" against it, but I just don't like the idea of a slide slamming shut on an empty chamber because the recoil spring is designed to strip a cartridge off the magazine and load it into the chamber . . . thus when there's no cartridge to load the entire spring effort is directed to slamming the slide home. Just seems like a little abuse I can avoid by letting the slide down gently. Takes no effort or appreciable amount of time. Makes me feel better.
JMHO

JMHO