I think the "theory" was that on a CDR that's barely readable, the laser will have to track more often, leading to more work, leading to faster breakdown. I guess if you have a bad burn, and can actually hear your drive having to retrack often, then don't use that disk. If it works fine and sounds normal, probably not much chance of damage.
Edit: Also, I would NOT recommend trying to use an older burner just to get slower burn speeds. I've tested two different 1x burners years ago, and the newer 8x burner I had always produced better disks. Slower does not always equal better. It's mainly the quality of the burner itself.