I'm unsure whether to book some hotels/hostels before I go. Although my itinerary isn't set in stone, I'm worried that I'll get somewhere and there won't be anywhere for me to sleep!
Am I better off booking somewhere and then cancelling it if my plans change? or should I just wing it
I'm travelling to Thailand, Laos, Cambodia and Malaysia
Time of year and preference can be big game changers on this one. If you were going to KL and other parts of Malaysia during Chinese New Year I'd highly recommend booking ahead. The same could be said for major festival or regular public holidays in any country.
If you find a hostel or hotel that is exactly where you want to stay - like Babel Siem Reap - you may want to book ahead simply to secure your stay there, as the highly popular locations often fill up fast. Also, if you already know the area you want to stay in the town, then it can be advantageous to book ahead if there are a limited number of accommodations there - like the riverfront doped up spots on the northwestern bank of Kampot.
However, if you are slow going and plan to spend extended periods of time in certain areas, then you can always arrive and get a place somewhere -- within a day or two you'll probably get an opening at the place you later scoped out and wish you had stayed. For slow traveling I think it is fine not to book ahead.
Finally, it doesn't hurt to have a few milestones where you know exactly when and where you want to be. If there is a certain location you really must check out a few weeks into your schedule, say that specific bungalow operation you heard was just perfect on Lonely Beach of Ko Chang, then book it and get there on time to save the lose of deposit.
You've got options.
Except for during peak holidays you can almost always find somewhere to stay on a day-by-day basis. You might just want to pre-book the first few days upon arrival, until you settle in and decide where to go next. Pre-booking all your accommodation in advance means you have to follow a prescribed route and schedule which takes away most of the fun. For the most part, wing it!