labingi

Interesting question. I guess the safe answer is "ask your lawyer." In general, I think you're fine using a common phrase that happens to be a quote if it has no bearing on the source of the quote. Otherwise, much of the English language would be copyright violation. Example: "I'll wait for you," is a line of out "With or Without You," but no one would say that you can never have a character say, "I'll wait for you."

In the specific instance of "All you need is love," I feel like hedging a little more because the words are so iconic "Beatles" that in almost any context, it might make people think of the song.

My best advice is to read individual publishers' guidelines about what they will or won't accept. I hope that helps a little!