You increase your risks of dropouts and sometimes audio problems. However sometime back someone posted and bragged they had reused a tape 21 times without any problems. I don't know, though. I wouldn't risk it, especially if you are taping something important, especially a job, then use a new tape.
I also wouldn't use anything other than 63 MQ, the PQs are fine, but find one and stick with it, don't change.