Yes, if you want the forked coins, you should withdraw from any site that will not issue them.  Personally I only have coins on Nitro and I have been burned by Nitro twice before on this so that's why I thought of them and wanted to warn people.  I think your 99% estimate is a bit overboard.  I know Fairlay and SWC Poker paid out bitcoin cash to their customers so it's far from unheard of.  Nitrogen is probably the most popular BTC gambling site so it's something I would expect from them considering smaller shops were able to do it.