Assigned role> pick order.
Pick order was always meant to be used as a last resort when communication completely failed. Now that communication is done prior to queueing this issue is resolved. You agree to play one of the two roles. You are assigned the role. Taking a different role without the consent of the person assigned said role is toxic.
Pick Order is meant to be a last resort when settling discussions - this is why we are endorsing it and not mandating it. Communication and collaboration are still the most important things when it comes to playing ranked games, and we hope players will continue to uphold the Summoner's Code in all of their matches. If you refuse to communicate with your team and rely only on Pick Order to determine your role selection, you are still being a toxic player and this will reflect negatively upon you. By officially endorsing Pick Order, however, we are hoping this will give more structure to players and may help settle some disputes. And, of course, this isn't the only thing we're doing to address champion select as a process.
http://forums.na.leagueoflegends.com/board/showthread.php?t=3449932
This was a post from a long time ago explaining pick order. With the advent of the new system arguments over role should be concluded and no longer needed.