if they have ink stamped them then they have been cancelled? so it wouldn't be a problem. The point is if you tape over them they can't stamp them to cancel. If they are only partly covered then I doubt it would be a problem.

I pass over the counter 20-40 envelopes most days they ink stamp every single one.

We put a blank label on the plastic coated envelopes and put the stamps on that - never had a problem with stamps falling off.