We arranged tour guides in advance of our short visit to Siem Reap but actually think it was a waster of money. We found the guides from TA reviews but the first one, Ratanak Eath, having confirmed that he would act as guide sent us an email about 10 days before the trip wanting to change our carefully planned schedule to accommodate some family ceremony that he wanted to attend. We dropped him and went with Kim San who guided us for 2 days and then told us that he couldn't take us out on day 3 as he had some family matter to attend to. They are pleasant guys but have an issue with committment!!
Having done the trip I wish that we had waited until we got to the hotela dn then planned it from there. If you stay in Siem Reap the main temples are very close by and many people just hire bicycles and ride there. The area is the flattest place I've ever seen so cycling is easy. There are a couple of temples that are a little further away but there are literally hundreds of tuk tuks that would take you there and back and you also use them to get around town for restaurants, markets etc.
People in Siem Reap are wonderfully friendly and willing to do anything (unless they are licensed tour guides, of course) to make your trip memorable. A fantastic place and easy to fall in love with.