I think it varies. My house, if I were to sell it, would list at about $115,000 and probably sell for about $105,000. Zillow says it's worth $79,000. That is taking into account the recent sales of 2 disaster houses that were purchased in the last couple of years and my county's tax assessment (which declares my home to have 200 fewer square feet and one fewer bathroom). On the other side, within the same zip code, I looked at a home that Zillow declared to be worth $285,000... it ultimately sold for $210,00, which is exactly what I'd have offered.
I'd trust the local real estate agents to get this number close, by and large.