Depends a lot on the popularity of the hotel, and the season. I've been to Negril during slow season and not pre-booked and had my choice of rooms at different hotels. Often paying less than advertised for cash in hand. I've also had to book 9 months in advance to get a specific room at a popular hotel during high season. I always tell people, unless you're going during slow season and aren't particular about where you stay, you're better off pre-booking. Or at least calling a lot to see if rooms are filling up! If you want a specific room, a specific hotel, or to travel during high season (or a holiday like ATI), IMHO you're better off pre-booking.