Rarely
Very rarely will i ever show cards if not paid for. That being said, it all really depends on the table. Sometimes you have a table where you might want to get a little credit in the bank for playing solid, so show one or two 'premium' hole cards that you raised. It may let you get away with more later on. Same goes in reverse if you want to get called on your raises, show a few bluffs early. Works muuuch better live play. Quite a few online donkeys don't seem to care or even pay attention to hand histories...