I think knowing when to show is very simular to figuring out the playing style of each player at the table. You have to look at everyone there and decided which is better to show, the monster or the bluffs and which will be more benificial later on.
If you are playing in a very aggressive game, show the monsters. Gain the respect of them so you can later bluff at a pot when you know they don't want to call. This doesn't works well in a very passive game with callers only. Show the monster hand and you lost the power of the caller. They will give you too much respect and you won't maximize your hands. You don't really want to try and steal in this type of game because you never know when you will get called.
This will work on very tight games when you are getting heads up after the flop. When there are only two of you playing, the odds
of flushes and straights hitting are less so you can attempt to steal with scare cards.
Showing these bluff wins may enable you to get that call on the river when you do have the monster or caught your flush draw.
You have to learn which type of person will use the knowledge you just gave them to your
advantage. You sit down at your computer or live table and start looking for the signs of the person that you might be able to take advantage of later.