The idol is only valuable as long no one knows or even suspects you have the idol. I think once someone knows or suspects you have it, you should just play it even if you think you dont need as it just puts a big target on your back.

A lot of people think that yul's use of the idol was the correct way to play an idol; but in reality his idol was different in that season as he could play his idol after the votes were read - so there was no blind side risk to yul. I remember at the final tribal council where yul said the biggest risk he took in the game was getting johnathon to switch through the idol - however, this is laughable as what risk did yul really take?