It's usually OK to re-use a MiniDV tape several times. IN the olden days, when the signal depended primarily on the amount of magnetism in the tape, every time you passed the tape over the heads you lost some of that magentic capacity. With digital, it's not as direct relationship; you can loose a bit more of the tape coating and the electronics can still recreate a good image.
BUT, I (nearly) never reuse tapes; for the same reason as Delphis and boBQuincy; MiniDV tape is a very inexpensive storage container (less expensive than storing the digitized video on a hard drive, usually). Also, (with my memory these days) I'd rather make it standard procedure to use a new tape each time and avoid the possibility of recording over something important; probably something that I haven't captured to disc yet.
---Ritch