I have only been to Myanmar and Thailand, but I didn't need any Guides in Thailand (only on places where you can't get in without guide) an in Myanmar also only for a few places. 
In general you don't need one, however if you are going for a hike I would recommend it strongly. A guide may be expensive, but they can tell you tons of stuff that you can't find in the internet or guide book. 
If you have the money take one from time to time :)