From Qxx, I lead third in general. But if partner has overcalled, I have no outside entry, AND I think there is a chance that dummy might come down with Kxx, then I lead the Q.
From xxx, my preference has always been a high card suggests I have nothing to work with in the suit and a low card shows something. Particularly with partners who do not look carefully at the spots in dummy and their own hand, I will sometimes break fourth best agreements because 4th best looked too much like a high card or MUD agreements because the middle card looked too much like a small card.