Or should they go back on their word?
Depends on the promise. What I find though is that promises to do good things (i.e cut spending, close gitmo, bring our troops home, cut taxes) are always broken while promises to do bad things (i.e maintain current welfare spending, add insurance regulations, use the government to send more kids to college) are always kept.
Should be the opposite. If a candidate promised to bomb Iran and then went back on his word, this would be great. If a candidate promised to cut the income tax to 5% and then kept his word, awesome! (And then someone should tell the guy to cut the rest).
Unfortunately, you can tell whether a politician will break his promise or not by considering the morality of the action promised.
You get the point.