I've had a few instances where there were white specks/dust, and while most of them were wipeable/blown away by air, some of them can't be removed without disc-marring force. It's not common, but also not rare.
Mostly I just try and see if I get good burns out of these "dirty" discs, simply because I'm a cheapskate and I want to get my money's worth. I get lucky most of the time since they don't affect my burns that much. YMMV though.