It occurred to me that I never answered the original question.

If if I shut down the engine for however long I do a run up. Lots of funny things can happen during start up. It only takes a few seconds and it makes me feel better. That is not to say I have never stopped an engine, restarted an took off without a new run up. There are certain circumstances where I will skip it.

If I don't shut down the engine -- like land, taxi back and take off again? No, I don't do a new run up. I am sure that someone will be able to explain to me why I should. If so, I will listen attentively.