If you truly do not care where your cabin is on the ship, let Royal pick it. If it matters what deck, view, elevator proximity what’s above or below your deck, or any other thing, select it yourself.
I select my cabin 99% of the time. The few times I have let Royal pick were fine but I’m not picky and don’t spend a lot of time in my cabin.
I see a lot of posts from people who take the guaranteed cabin rate then complain when they get their assignment. So if you think you would be disappointed with any particular area of the ship, don’t book the guaranteed stateroom.