You are discovering more than you expected in the answers. Steve is correct, around the country there are different ways information can be provided with different levels of accuracy. I like Trulia as an agent/broker because of this side of the site. Questions are asked and various answers provided for the public to get a sense of the quality of the agents involved as well as an answer to a question.
When you are ready to collect off the data available, and especially in the current state of our local market, there is nothing that competes with a local agent setting up a search matching your criteria and having it send the results to you in real time.
As others have said, many IDX sites only update periodically. (IDX is the means that Redfin, JLScott, Windermere, Remax as well as individual agent sites get their mls data) A live search set up by me or another local pro can have a listing in your email within minutes of it hitting the market.
With inventory so low and demand so high, time matters. I'm seeing new listings in the morning being under contract by the evening, before many can get a chance to see them.
A local broker can also inform you about known upcoming listings before they hit, search canceled and expired listings that may match you parameters and if necessary solicit specific homes that appeal to you that are not on the market.
Get a great local pro and you'll be ahead of the crowd and improve your odds of finding what you are looking for much better than with this or any other site.