Zillow is all over the map. They are too high on the Bay Area properties and above, below, or near value on the other rentals. You can elect to use Zillow in Personal Capital to calculate your net worth including your properties. Those values can change almost daily, based on Zillow's mood.
Just for laughs, I went through their buying process on one of the vacant rentals. We had seen a couple where they had goofed badly and overpaid, so what the heck. Do paint and flooring, clean up the landscaping, and this one would easily go for $250k. Zillow estimate was $240k. Offer sight unseen was $222k, net $206k. The answer to why the offer was so much lower than even their estimate? "We had an agent in the market look at comparable sales."
In the mid priced areas of the Phoenix market, demand for under $300k houses far outstrips supply. Houses sell in 48 hours, sometimes with multiple offers. Prices are going up rapidly. I don't know how Zillow incorporates that in their valuation model, but their value estimates are increasing. I know what my houses would sell for better than most experienced agents, and I use my numbers in estimating net worth.