Here a couple of the wisest things I've heard in a while:
"The best bit of parenting advice I ever got was that some battles aren't even worth starting never mind winning!" from Sally at Aunticookie.
I think this applies to everything, not only parenthood. Ultimate war strategist Sun Tzu used something very similar as a personal credo, and a lack of discern on Napoleon's part when choosing which battles to fight(and which ones not to) started the beginning of the end for his empire.
"Formula for a happy marriage: Keep the fights Clean and the Sex dirty" Kyra Sedgwick. She and Kevin Bacon have been married for over 20 years, which I think equals Eternity in Hollywood standards.