[pzhon]

First, this is a type of situation in which you can bet the river for value even if you don't think you will win 50% of the time. People will call and overcall with many weaker hands because the pot is huge, so you expect to put in less than 50% of the money even though you are prepared to pay off a raise.

Second, you shouldn't be as scared of a backdoor flush hitting as when someone could have had a flush draw on the flop. This is basic hand-reading.

Third, you should be ahead well over 80% of the time. I think failing to bet will cost you more than 1 BB on average. If someone had 54 for the straight, he made the nuts on the turn and didn't raise you with a vulnerable hand. Even passive live players will often find a raise with the nuts.