This week I am beginning the last of 21 aligners: one tooth at the bottom still isn't straight.

I typically add 3 overcorrection aligners and usually use the first two.  So I would wear them, especially if one tooth isn't straight.....but....your Invisalign dentist needs to advise you on this to make sure that tooth is actually not straight or if the incisal top of the tooth is just uneven as is many times the case.

MrsSmile1, Your overcorrection aligners will close any residual gaps. They will not correct teeth which have not fully expressed their movements. You will need additional aligners. The need for additional aligners is not unusual. Some types of movements are more difficult to accomplish using aligners than others. Dr. Neal