If they have one of the lower categories at a good rate under what is called a "Guarantee" or "Run of Ship" status, it's usually the best rates and increases the odds you'll get upgraded. Look at one the the other threads on here that's titled "Princess is the Best!" That was an upgrade from a guarantee. What they do with a guarantee (if it's available) is offer you a cabin guaranteed to be at or above the category you book but allow them to randomly assign the cabin after all the people who want to pay a little extra to choose thier cabin have booked. There are typically more guarantees sold than they actually have cabins in the lower categories. Sometimes they leave the guarantees open even when they are out of standard balcony cabins all together. In those cases they're simply trying to fill the ship. Since you're guaranteed a balcony or better, that means someone is going to get moved up to a suite, as happened in the case of the thread I mentioned. One drawback, if you book a Cat BE guar (just an example, not saying to book Cat BE) you may well get a BE. You can't choose, so you may be by an elevator. You may be at the front or rear of the ship or even under the disco. Booking a guarantee is a gamble. It often saves money and often gets you more than you pay for. But you could get a cabin you wouldn't have chosen. I do it often for myself, and often for clients after giving the above disclaimer. The vast majority of the time you will be pleased with what you get. Once in a while you'll get something you didn't expect, exactly what you paid for. A low category cabin, not in a premium position.
If you want the cabin in a premo position and to eliminate the probability of an upgrade beyond your expectations, pay the difference and choose a Cat BA or BC midship on a high deck.