I'd recommend it. If Coinbase is set on sticking to the original blockchain, in the event the fork prevails, that means your money effectively becomes unusable - you don't control that wallet, but Coinbase won't submit your transfers to that forked blockchain. The money dies on that chain the moment Coinbase decides not to interact with it.

If you have it on your own, you can technically use it on both chains independently. The history breaks August 1st. From there are on it is basically two currencies.

Though in general you should keep your bitcoin close to home. If Coinbase gets hacked or just ceases operations your money could just be gone if the fallout is lost in litigation hell.