I was taught 15% pretax growing up (1980s), but clearly the general expectation has changed over time. Most people consider 20 the standard now, and many seem to think even differentiating between pre- and post-tax makes you a dick.
Personally, unless it really sucks, I err on the high side because servers are underpaid and I like to be remembered well, whether I'm a one-time visitor or a regular. Bad service, probably still leaving what my mom taught me. I still get paid if I fuck up at work, and I don't need my pay docked to make me try harder next time. I'd have to see pretty wretched service to go under 15%.
I do think shit's out of control with the trend toward less and less guaranteed wage and more reliance on tips, because research proves quality of service affects tipping somewhere between "a tiny bit" and "not at all" for most patrons. That is, the assholes stiff the server even if they kick ass, and generous tippers usually overlook bad service and leave 20-25% most days. Add in the fact that servers are taxed on presumed tip percentages that they may or may not actually earn, and the system is pretty fucked up. I would support substantial reform if it stood to reduce the uncertainty, both for wage earners and customers.