The Zillow values are usually inflated, based on my experiences. But they give you a starting point. If you dive into the Zillow site (or any of the realty websites), you can see what the houses around you are being listed for and what the recently sold houses were valued at. Use these as a basis more than the Zillow estimate. You can adjust up or down based on the features and condition of your own home. Sometimes that number will come close to their estimate and others it will not. Use a bunch of common sense when you look at their numbers because sometimes they will pick up something that makes no sense (common in in-town situations where you might have a million dollar house next to a less than desirable neighborhood).
I personally use the last appraised or purchase price in my own net worth calculations. That's pretty conservative but at least it has documentation to back it up.
Mint uses Zillow in its calculations. I figure the true number is probably between my ultra-conservative number and Zillow-based number.
But here's the thing on net worth estimates - it is just an estimate. And if you aren't using it for something like underwriting on a loan or business proposal, it is ok if it is off. Especially if you know it is off and can realistically look at it and adjust it mentally. So if you are using Mint or another program that uses Zillow to automatically calculate your net worth for you, is it better to have 100% accuracy or to know the trajection your net worth is on? When I'm using Mint, I just take $20-30k off my net worth calculation mentally...