Well, I recently discovered that notes taken on your iphone prior to enabling iCloud sync will remain on your iphone regardless whether you have your iCloud Notes sync set to on or off. The reason is that once you enable iCloud notes sync, a new account will be created for your already existing notes called "On My iPhone". More details about this, you can read here: Unable to sync iPhone notes
Notes which will sync over will only be the new notes which you create after having enabled iCloud. Unfortunately, you cannot move notes between accounts. This is one of those peculiar Apple limitations which only serve to create a clang of chatter around their products among the Apple community. I wonder if they do that on purpose! :)