if one midrange needs a new foam surround, should

Yes, I am having some woofers foam surrounds replaced and the guy doing it said he was always having people bring one in only to have to bring in another shortly after. If you do them at the same time you might get a better price but you will certainly save time and effort and make sure they sound the same.

Agree with Stanwal. Besides physical condition, it is best to re-foam both sides at the same time to maintain balanced sound quality.