C / C++ compiler warnings: do you clean up all your code to remove them or leave them in?

I would clean up any warning.
Even the ones that you know are harmless (if such a thing exists) will give a bad impression of you to whoever will compile the code.

It one of the “smelly” signs I would look for if I had to work on someone else code.

If not real errors or potential future issues, it would be a sign of sloppiness