Zillow is only as accurate as the information it has, which is often based off county/city info. If you've checked your own home and know there are problems with the information, you should claim it and make the appropriate changes. I know that the info it has for both our properties is correct, but if it wasn't I'd make the changes.
As for home values, I don't think that they are very accurate. Our primary home's value has gone up and down over $50K four times in less than 10 days. Also, we own one of the smallest houses in our neighbhorhood, yet it is valued much more than some of the other houses.