What Rick said is true, if you want a specific cabin, then book that cabin, if you really don't care that much, just book a catagory guarantee, but book the catagory you will be happy with, if no upgrade is given. You are a bit more likely to get an upgrade by booking a guarantee, but that is not a guarantee of an upgrade. So far, we've booked guarantee cabins twice and not an upgrade yet
With booking a specific room, you get that room. I always book "guarantee". I've been fortunate. I got bumped from lowest inside to an outside window on Celebrity, on RCI I've gotten a 2 cat bump and a 4 cat bump, on Princess 2 cat... However, usually I get bumped to the front or rear of the ship. These areas will feel more ship movement than dead center ship. BUT ALWAYS, book what you will be happy with, don't book below what you will be happy with...and also, a bump isn't a sure thing. Don't go into it expecting a bump, and then be disappointed if you don't get it and let that color your cruise negatively. I look at it all as "gravy", and I've been fortunate so far.