Is it okay to record over a DAT tape ?

Yes. While a DAT machine does not erase the tape before re-recording, it does do a good job with this. When doing an alignment on a DAT machines record heads, you must use a fresh section of tape as the old signals are still present and can confuse the technician. We have never seen this cause a problem in playback. We do, however, recommend fresh tape for mastering applications, save the used tapes for dubs.