Do you know anyone who actually works in food service? I do, and they'd be pissed if everyone skimped with a 15% tip. It's 20% for adequate service, 25% for excellent.
Taxi drivers get $1 no matter the fare. The gall of that industry to even ask...
And even the slightest infraction of protocol and I leave 0.
I used to overtip all the time. I got absolutely nothing of value for doing it, so forget it.
I'll tip 30% at like an Ihop when my bill is 11 bucks because "total tips" is what matters.