The only guideline is that the actual chance of winning the wager be higher than the implied odds. For instance, at -300 the implied odds of that team winning are ( 300 / (100 + 300) ) = 75%. If that team actually has an 85% chance of winning, then you should absolutely fire.
Conversely, if a team is +500 and only has a 10% chance of winning (implied odds of ( 100 / (500 + 100 ) ) = 16.67%), then you should not make the play.