Not always. In the 2004 election, Kerry won in exit polls but lost in the official count. The discrepancy was especially large in Ohio--which, had it gone as predicted by exit polls, would have resulted in President Kerry.

A few days ago there were big gaps between official and exit poll results in Arizona, favoring Clinton over Sanders.

I wouldn't normally link to Democratic Underground, but this post has a great summary of exit poll discrepancies, voter suppression, and the evidence for election fraud both in the 2004 presidential election and in the current Democratic primary.

http://www.democraticunderground.com/12511568043

This is a very urgent problem that I think the tech community can help fix. We need open source, auditable voting machines and professionally run elections. Election fraud, or even just the appearance of fraud, is toxic to democracy.