For me though, I always go with my Khmer friends to the ones in a market, simply because it is cheaper - US$1 for a hairwash and $1 for a hair cut. I also like the service. The best hairwashes are done by the ladyboys because they are strong and passionate about their job. You can find them working at stalls in the Old Market.
My friends and I favour a small beauty shop in psar dum krolyiang (Krolyiang Tree Market) here in Siem Reap. It is a local market and not centrally located so tourists do not go there.
Here is the shop. You can see the ceiling is padded with cardboard boxes. I asked why and it is to keep the place cool since the shop, like all the others, has a tin roof. In front there is a plastic sheet to keep the rain off. The sheet is all green and mouldy.
These are the sockets. Could be disastrous if something goes wrong with the electrical wiring, I guess, since the shop is just a wooden shack. But they are careful as no one here has insurance.
With this! A giant 60 litre pail filled with water. The hairdresser scoops the water and pours it over your hair. (Yes, no warm rinses either. Only cold water). I thought it was great. Saves a lot of water this way. The water is bought from peddlers who go around the market (distilled water is cheap in this country - just $1 for 20 litres and one day I will show you why).
I have to point out the beauty shops in the Old Market in heart of Siem Reap do have pipes and therefore running water. Not all Cambodian beauty shops are as basic as this.
I know many friends who balk at the idea of getting their hair washed in this way, but I've never had any problems (no itchiness I mean). Try it, the next time you're in Cambodia!