When you tip, do you pay 20% (or 15%) on the sales tax portion of your bill? Or just on the untaxed portion?

Here's an example to clarify my question. Say your bill was $50 plus $5 in sales tax for a total of $55. If you tip 20% of your bill, would you pay the 20% on the $50 (for a tip of $10) or the $55 amount (for a tip of $11)?

Is there a standard here?

The general rule I've heard is that you "don't tip the tax or tax the tip." Since the sales tax where I am is 7.75%, doubling the tax amount will give me a good estimate of what kind of tip is appropriate, adjusted for quality of service. And I never tip less than $1, even if all I had was a cup of coffee (as long as it's at a sit-down restaurant, although sometimes I'll fling my Starbucks barista a buck if I'm feeling generous!) Luckily, I've never had service so bad I thought it warranted no tip at all.

Just the food (not the tax). Why pay 15% of government add-ons?
im a waiter on i still pit on tax. 10% of what you eat we get taxed on at the end of the year. i always tip good unless the service sucks but i automatically always tip 20%
Well the untaxed part I guess, although 20% of a 6% sales tax is basically just a few more pennies added anyway, so no biggie. I like to divide the dollar part by 5 and then round up to make the tip just a little over 20%, plus a little extra if the waiter/waitress was really nice.
I usually pay 20% of the total, including tax. That's my norm.

My husband was a waiter, and most of those guys work their butts off for crappy tips.
It's only when I get appauling service do I tip less.
But I still tip.
don't tip. they get paid to work anyway. i don't get tips at work and as a joiner i work damn hard, so if any one deserves a tip its the people sweating and bleeding for their money not some one walk in round a restaurant in an easy job!
I pay 20% of the total bill. I don't recall tax on my food bill (I guess I don't pay that much attention). The service is usually worth it tax or no tax.