National polling gives you a good idea which state polls are bogus and which ones are accurate though (eg: if you believe national is tied, no way is Clinton up 4 in GA like a poll that came out yesterday; alternatively if you believe Clinton is up nationally by 5, no way is Trump up by 4 in Ohio, 5 in FL and 5 in NV like a poll said yesterday)
I disagree. National polls project the popular vote. State polls project the Electoral vote.
Al Gore won the popular vote by about a half a million but he lost the Electoral College and the presidency by 537 votes in Florida.
Its not so much how many votes a candidate gets as it is where they get those votes. Donald Trump might win Georgia by a million votes, he still gets 16 electors. Clinton might win California by 1 vote, she gets all 55 electors.
Taking an average of several most recent state polls is a good way to get a realistic sense of the state the race in that state. I never trust any one poll, state or national.