Because we are one of a handful of non-disclosure states, Zillow and Trulia are not always accurate, but they are improving regularly as time and sales go on. They are probably the best publicly accessible sources of values, regretably. If you are buying, you will know a good value when you see it after you have been out to see enough homes. In my experience, that is about 12-15 homes for people who have been shopping online and working with me. Once we have seen what we both feel are the best values that meet their criteria, they know well which ones are the best out there and when they see the right one. Also, when you are doing the legwork, you should not worry about "bothering" your agent, as that is their job and it does not take a good agent long to provide you with useful information from the MLS - and that is after all the best source of data. If your agent seems to get bugged by your inquiries, you should probably consider shopping for better service. Some brokers, like me, even negotiate a commission rebate for buyers who assume lots of the legwork and save me time. If you are selling, you really need to compare to your direct competitors that are also active in MLS, so make your agent jump in the car and show you some of those. A good agent is never burdened by a diligent client. Best of luck to you Erik!
NM is a nondisclosure state so the Multiple Listing Service has the most accurate data. What you find through public websites such as zillow.com is not accurate. They give you a rough estimate to get an idea. Appraisers use MLS data along with other tools to come up with a value. To find the best comps look at what has sold for the last 90 days. If there are comparables in your neighborhood I was told by an appraiser I talk to that they have to use those. They also have to use any foreclosure and short sale data they find within your range. It is not an exact science but by working with a realtor you will have a knowledgeable partner that will help you get the information you need.
Any of those online services is going to be somewhat inaccurate and incomplete.