One Hand Clapping has some nice thoughts about tipping. He starts with an extremely small sample and observes that a significant sample of those in that subset who are not tipping are Christians and concludes (via a clear logical fallacy) that Christians tend not to tip (or not to tip well). Just to be clear on this, let's look at the structure of the argument.
1. Those who tend not to tip are often Christians.
2. Therefore, Christians tend not to tip.
I see no way to conclude 2 from 1. You'd need to look at most Christians to see if most Christians tip. You don't start with the group of people who don't tip and then notice that many of them are Christians. There may well be a much larger group of Christians who tip and tip well.
Aside from that obvious mistake (which many people in the comments also made about Christians and other groups), there's plenty of good stuff in this post, mostly on the ethical reflections. I wish he wrote more on everyday ethical issues, because he has some great wisdom on these matters, and he often writes about issues most people wouldn't think to include in an ethics book.