My friend Rocco and I were sipping cappuccino and resting our aching feet after a grueling night of waiting tables at the Positano Restaurant in Bethesda as I tried to assure him that Colman McCarthy, writing in this space last Saturday, had our best interests at heart when he urged people to stop tipping.

Rocco, my patron in a Plimptonesque adventure as a part-time waiter, shook his head in puzzlement as I explained that some liberal thinkers such as McCarthy believe that, if people stop stuffing money into every outstretched palm, it not only would make better people out of the un- tipped, but also would help the econ omy.

Perhaps it was a language barrier that prevented Rocco, whose wide smile and sure hands have graced the finest tables in New York, Washington and his native Italy, from understanding McCarthy's motives.

Had Rocco so soon forgotten how we had to destroy Vietnam to save it? I asked. And surely, Rocco, you agree with President Reagan that it is necessary to cut food to the poor and taxes for the rich to bring our nation out of its economic morass.

If customers withhold gratuities, economic pressure will force owners of restaurants, beauty shops and parking garages to raise the pay of their employees, giving the workers bigger paychecks and the government more taxes, I explained.

"But who would pay me as much as I can make in tips?" Rocco asked incredulously.

"My man, you miss the point," I said, trying not to lose patience.

"I guess I do," Rocco said stubbornly.

By then, our conversation had attracted the interest of Luciano, Anto