Hi Fabian,
It is only really recommended to freeze tapes due to a fault. To prevent them being used you can suspend them, and to prevent expiration, change the epiration date with the bpexpdate command. Freezing a tape will not prevent the images expiring, and once this has happened the tape will expire if unfrozen.
If NBU freezes a tape, it is because it believes there is a technical issue with the tape, if you have a number of other frozen tapes it can be difficult (impossible sometimes) to work out why they are frozen.
If you really want to freeze tapes, write down when and why, so that the real frozen tapes can still be identified.
Martin
