Hot on Reddit is a pic of a receipt from an unnamed restaurant in which the patron was unhappy with an included 18% gratuity, complaining, "I give God 10% – why do you get 18%?"
What really grinds my gears about this sort of thing (aside from the whole stiffing your wait staff thing) is that nobody gives their money to God. It's impossible, because God does not exist and, even if he did, he's "supernatural" and cannot physically take your money. So what people really mean when they say that they "give 10% to God" is that they give that amount to their church. That generally pays for the salaries of church staff, capital investments for church property and equipment, and – if you're lucky – some charity work.
There's no magic at work. If you want to give to charity, it'd be far more efficient to just give it directly to charity than to give it to a church. And your money isn't supporting "God", but the propagation of religious dogma that quite frankly our world would be a better place without.