Public opinion polls about politics

We often see public opinion polls about politics including, but not limited to, polls that show which candidate is “winning” or “leading” before an election. (I put “winning”  and “leading” in quotes because until there are actual votes we probably shouldn’t use the words “winning” or “leading.”)  Can we trust the results of these types of polls?  If so, why?  If not, why do media outlets bother reporting them and why do people bother paying attention to them? Should elected officials ever concern themselves with public opinion polls (or any other measure of public opinion for that matter) when making policy?  Or, should they just do what they think is right regardless of what the people think is right?  Explain.