From a person whose accuracy on this I can vouch for:
The L.A. County tax assessor lowered our assessed property value for 2010, but not for 2009. (We’d prefer if our house kept gaining value, but if it does lose value, we’d like to see that reflected in a lower property tax bill.) We decided to appeal the 2009 decision, and when I showed up to the hearing, I learned that the assessor’s office had lowered the assessed value by $140,000 — something it hadn’t done before we appealed. I presented some of my arguments about why even more should be taken off, and the hearing examiner lowered the assessment by another $50,000. This means that, at our 1.22% property tax rate, we’ll be getting a tax refund of about $2300.
Now much turns on where you are, how excessive the initial assessment was, what kind of hearing examiner you draw, how honest — and how aggressive — the county tax assessor’s office is, and more. But this experience suggests that at least sometimes you can prevail even over the tax man.