Do you have to show your hole cards when playing the board in Texas holdem at showdown after everyone has checked and you are in 1st position and declare "I am playing the board"?
Yes. And be careful, a friend lost a large pot at a casino this way.
He held A9 against KK all in, and paired his ace on the river. The opponent was trash talking, and in the heat of the moment my friend mucked his 9 (since it wasn't needed to make his hand), tabling just the Ace to demonstrate his win.
But the opponent objected, and the floor manager confirmed that my friend was required to table both hole cards. Friend pulled his 9 out of the muck, but that wasn't good enough - mucked cards are dead and you have to table both hole cards to claim the pot. It was a hard lesson.