If I am strapped for gold per turn, my usefully promoted units will be kept for future upgrading, while all the rest will be sacrificed, usually right down to what's necessary for happy caps. The exception is leftover siege, which I never destroy for any reason. Leftover mounted units are nearly in the same category for me.
If I am not strapped for gold per turn, I keep all those leftovers and position them to move in after I have taken cities to prevent revolts.