This wasn't addressed in the weekly thread for either–in Southeast Asia, you can stroll into a town at 2am and a tuktuk will take you to a guesthouse, any time of year. Reserving a room online is for suckers and will cost you more. In Australia, hostels are frequently booked out and it's frequently cheaper to book online in advance.

What's the story in Colombia and Panama in Jan/Feb? Should I book rooms ahead online or will I find better deals walking around the backpacker areas?