Booking in advance is one of those things that brings you peace of mind, but that is not necessarily awesome for your wallet.
In El Nido, Palwan, we stayed at the best resort ever, Stunning Vistas. For the three of us, we paid a total of $500 for our stay. That included: two full day island hopping tours; a half-day kayaking trip; a clean, comfortable three person room with a fan; breakfast everyday; lunch most days; and beers.
We didn't book in advance and the rate was about 400 pesos cheaper per night than what they quoted me in the e-mail. After our first evening, we decided to stay there for the next five nights. They only had the one room available (the other was a deluxe room and about double the price and size) and just happened to have vacancy for the next five days. The morning we were leaving a group would be checking in and staying at the resort for almost 2 weeks.
Talk about luck!
In Sabang, the bottom of Palawan, we stayed in another amazing resort. This place was a bit more rustic and I found it on a really great blog...that I haven't been able to find again. It was a Filipino woman who was backpacking alone and often. Anyway, there's also a lot of information on this blog: Lakas.com.ph Anyway, so this place is called the Blue Bamboo Resort. It's traditional Filipino housing. Very affordable and a beautiful location. I doubt we would have been able to find it if I hadn't e-mailed the owner in advance to book. The three of us stayed in a bungalow with a traditional bucket flush toilet and rain water showers (cold as can be) for just over twenty-five dollars an evening. They also had a small restaurant. When we told them we weren't that into seafood, the owner walked around the corner, bought a chicken and killed it for us. That's service.
When it comes to Sabang, you have to bear in mind that most of the other places to stay are 3 or 4 star resorts. Their cheapest rooms were five times the price of our room... at least. Plus, you get to walk through the village where the locals live to get to the place. (It's beyond the basketball court; keep that in mind.)
Both of these resorts were locally owned and operated. Their hospitality was great and I would recommend them to anyone.
Booking Early = Guaranteed Rooms, But Usually a Higher Price.