In Conquests, any unit can be renamed; Control + N will let you do that. Not sure about vanilla and PTW.
After my units have generated a leader, I semi-retire them. I keep them in a seperate stack near my capital and then ignore them. If I can upgrade them I will, but most of my MGLs seem to come from Cavalry and they don't upgrade. I don't create this stack until things get railroaded. Until then, they need to be in the field in order to respond to threats quickly. Railroads let me respond quickly from a distance.
I will use the retirees to quell resistance in the turn that a city is captured; sometimes, if I think about it. The next turn they are back at the capital for ceremonial duty. If there are special combat needs that the normal units can't handle I will activate these guys. Special needs could be guarding wounded units or workers for the IBT or tracking down the last settler of an almost dead AI.
Recently I have started adding such units into Armies, to increase their hitting power. I don't mind paying them to do nothing but sometimes it feels better to have these elite units in action, earning their gpt support cost.