When, where, timing, and what cards is all up to the player. To me there is no right answer for this. If you want to show your hole cards a lot go for it, if you believe in making people pay to see then so be it. Either way I usually show for 2 reasons: 1) to earn some respect at a table 2) to irritate someone who is heading towards tilt after I bluff them. In this scenario hopefully my showing puts him over the edge