Hey Mathias,
I would attach a wasteboard over the entire surface, but it depends a little bit on which clamping method you use. If you use T-tracks (see also T-Tracks on QCW frame), T-nuts (Einschlagmuttern) or Insert nuts (Rampamuffen) that are attached on the wooden tabletop and not on the wasteboard, it’s not that important, but if you use the cyanacrylate and blue tape method, it would be important that the wasteboard cannot be bent upwards. The same applies should you attach any vises or clamps to the wasteboard instead of the underlying wooden tabletop.
Some people use clamps that sit in dog holes e.g. like these shown here, in this case I would make sure you use long enough dog holes that go through the tabletop and that the bench dogs are long enough, and it would be wise to have the wasteboard attached firmly to the tabletop everywhere since the dogs will have lateral forces applied.
Since it is a "waste"board (=Opferplatte), it would make no sense to glue it, as you will want to remove and exchange it once it is damaged or worn.