If you really don't care what deck and where on that deck your cabin is located, then book a guarantee, booking a guarantee sometimes increases your chances of an upgrade.
If you want a specific deck and desirable location then book a specific cabin.
Guarantees are definitely a personal preference. So far, we've always selected a specific stateroom, because that's where we wanted to be located on the ship and we weren't concerned about an upgrade (which is never guaranteed, anyway).
I have done this once before with Princess and got a 3 category upgrade. What I did was find the category that I wanted. It turned out that all the cabins were "booked" and just the guarantee was available. Then I looked at the higher categories and found open cabins. So I took a chance and got lucky.
They are "guaranteeing" that you will get at least that category or higher. So why not?
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08
We've gambled a time or two and it's turned out pretty well actually. One cruise we were upgraded from inside to bottom to oceanview on Promenade Deck. That sounds real good until you think about people walking around outside your big picture window! We had to be careful about "watching the stars", then another, we went from inside to Obstructed view window - which was actually not very obstructed.
BUT, if you have ANY problem with motion sickness, your best bet every time is to stick with inside room, mid ship. THese rooms have the least movement.
We're leaving in April for transatlantic and were tempted with an excellent price for a mini-suite on the 5th/6th deck , but after much thought, research, and discussion with other cruisers, we chose an inside mid ship room with a note that we do not want to be upgraded even though we've sailed with this particular line enough times that they may consider it.
I agree to book the minimum you will be happy with b/c we ended up WAY in front on a "GTY" with "RCI." "Princess" moved us from cheapest "inside" to the largest "inside" on a previous cruise and just assigned us a cabin on a "gty" we have in Nov (good location because all the "AA" cabins are mid-ship). Most of the time I do care where the cabin is since we love the aft cabins on the "Radiance class" ships.