In a cash game, you should be indifferent about taking a gamble. For example, if you think that calling a $5 bet will win you, on average, $5 then you should not care one way or the other whether you call. If you think that on average you'll win $5.10, it should be an easy call. The only reason not to make the call in the second case is if you really don't want to lose that money for whatever reason. That's a bad disadvantage for you, and you should play at lower stakes or not play at all.
If you plan on entering poker tournaments, there are smart reasons to pass up close gambles sometimes – that is, fold instead of call. The reason is that there is value to surviving, because that brings you closer to a higher spot on the pay ladder. Just how much of a profitable (in terms of chips) gamble you should pass up is a hotly debated topic.
