True... You may not know where others' hands have been, but you know where you...
Reminds me of the old joke about the sailor and the Marine taking a whiz in the public restroom (okay, diehards, in the head). They finish up and the sailor goes to walk out without washing his hands and the Marine sez, "Hey, Swabby, in the Marines they teach us to wash our hands after we go to the bathroom!"
Without batting an eye or slowing in his exit the Swab's retort is, "...and in the Navy they teach us not to piss on our fingers!"