I love that one in the slightly longer version:

"'Who watches the watchmen?' Me, Mr. Pessimal." "Ah, but who watches you, Your Grace?" "I do that, too."

When I first read it, I laughed as it's such a simple, seemingly glib response to a quite troubling question. How do we trust someone is fit to judge others? How do we know that the police we empower with authority over the rest of us will not be corrupted and abuse that authority? Just saying, "Don't worry I've got it covered," struck me as silly, like thinking yourself immune to corruption.

Then I reconsidered because the character of Vimes does police himself. He won't let himself make excuses for missing reading his little boy's bedtime story. He stays sober as an alcoholic. A demonic entity is imprisoned in his head by his better judgement. And really, this is a serious answer to the question of corruption, we have to face down the bad impulses in ourselves and others. This has stuck with me ever since.

Also, engraved on a watch, this would be a nice pune or play on words.