If you are paying for the session its completely reasonable to ask for a copy of the session files IMHO. If you are going to tape and they want to erase it and re-use it or something I would at least negotiate dumping it to digital.
I agree completely, but notice that many engineers and/or mixers seem to believe that the session/mix data (outside of bounced stems) is their IP. Which is truly stupid when you think about it.
