My advice about tipping in Cambodia is pretty much the same as most countries in South East Asia, tip if you think you got good service. It is not mandatory to tip in Cambodia but it sure is appreciated.
Cambodia is one of the poorest countries in the region and salaries are not high. A waitress or waiter in a restaurant or bar will probably only earn about US$120 per month in Cambodia. Even if you tip just a small amount, it will make a big difference to their monthly salary.
With meals and drinks being so cheap, many bars sell a draft beer for $1 or $0.50 during happy hour, so after having a few drinks or a cheap meal, it is not hard to add just a small amount for a tip. As always I only tip if I receive good service. There is no need to reward bad service.
US Dollars are still used extensively. If you pay in US Dollars they will give most of your change in US dollars and any cents will be paid out in Cambodia riel notes. With denominations of 100 Riel ($0.02), 500 Riel ($0.12) 1000 Riel ($0,24), 2000 Riel ($0.50) and 10,000 Riel ($2.40), it is easy enough to leave behind a few of the smaller notes as a tip. It all adds up.
Don’t forget to tip the house cleaning staff. Most of the house keeping staff in hotels probably only work in the mornings cleaning rooms and probably only earn less than US$5 per day. Read Do You Tip the Hotel Housekeeping staff