One other thing is that you might find yourself in a situation where you do NOT want an upgrade from a particular grade. I have just booked a cruise in an outside cabin but do not want to be upgraded into a balcony cabin, because (a) on that ship the balcony cabins including balcony are the same total size as the outside cabins without balcony and (b) the cruise will be too cold and possibly too wet for the balcony to be of any use. Booking a guaranteed cabin at an outside cabin grade might tip you into an "upgraded" balcony cabin that you just don't want, and I didn't fancy gambling on being upgraded umpteen grades into a suite. (I've also had to have the booking notated to show that I do not want an upgrade even if one is available.)