I would have thought that, as it's a very smooth surfae, it would be no problem to the fish and it's scales. The safest thing, I think, would be to make sure the inside of the bag is wet. I know this may add a very slight amount of weight to the final reading (a problem if you're going down into drams) but otherwise I wouldn't have thought it mattered.
I can honestly say that I weigh all my fish using a carrier bag and have never caused any scale loss that I know of, none being left in the bag after weighing.