Your ISP still has the ability to see where you route your traffic, regardless of it is to their DNS or to another open DNS provider like GOOGLE DNS or OPENDNS.
In either case, it is best to either a) set your DNS Settings directly on the router, or b) directly on any pc that doesn't wish to use your ISP's DNS.
Not really a matter of safety here; they are your ISP, they can see what you are routing; unless you decide to use an external proxy that routes your traffic only to that proxy and then out to other sites, the ISP can see pretty much anything you do on your network connection.
Note, they are NOT able to force you to use their DNS; that is your decision, and something you can directly override, but they can provide page redirection, etc, and inform you of DNS outages, changes, and such.