The thing about funerals is that it is part of the pastor's job to do them. There are many pastors and/or priests who have three or so fill-in-the-blank funeral sermons that they just alternate on because it can take some work actually learning the various relevant details of the deceased's life. I think the tip makes sense for preachers because they are the difference between a superficial funeral service and a relevant one.

I tend to zealously overtip.

If the service was good and there was nothing wrong, then they get a good tip. If I keep the few dollars, I'll spend it on something useless anyway. They could very well need the couple of extra dollars, and hey, even if they didn't, it makes them happier for a couple of extra seconds before they spend it on something useless.

If the server is being a total bag though, I'll leave less. If it's somewhere a tip is expected, I feel bad just walking away unless the person was a total jerk.

That wasn't really intended as advice. I was explaining my reasoning for why I don't tip in a hairdressing situation where it's optional. Part of it is that I'm paying a pretty good amount in the first place--usually around $70--so an extra 10-20% would really add on significantly to the cost. If I were paying at a Supercuts or something like that, where the price is as you've indicated, I'd probably change my policy.

Gilda