A few comments (and questions):
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(2)When 15% was the norm, doubling the sales tax (in most locales) was a quick way of figuring the tip.
(3)A waiter should prefer tips in CASH--less info for the IRS.
15% is still the norm. don't believe anyone who says otherwise. doubling the tax makes sense only if tax is 7-8%.
servers prefer cash not to avoid reporting, but because they get the money that day, as opposed to waiting until the next paycheck for CC tips.