It's one of those things that people who were raised right just do, out of common courtesy. But in this day and age that's probably out of the window.
I was raised pretty hard core Catholic. In church at least every Sunday and we never missed a holy day of obligation. My brother studied theology at the Franciscan University in Steubenville, Ohio. My whole family was/is very familiar with the Catechism of the Church and the rules. I'd say we were "raised right."
That being said, I've never once heard of tipping a priest for a baptism and would never do it or even consider it. Especially since once I reached the age of reason I've strayed from the Church, and religion in general. If I attended a baptism and I heard someone suggest it or the priest ask for it I would probably say something not kosher.