If an adjuster approved hail damage to my siding and ins.co. sent a check, do I have to replace the siding if I don't want to?

Most likely not, although you probably should. You should read your insurance policy and the letter that came with the check to see if there are any restrictions associated with accepting the money.  Best of luck! NOTE:  This response is general in nature and should not be considered legal advice.  No attorney-client relationship exists or is formed by this response.