This is a weird fear. I worked in restaurants in college and a terrible cafe in grad school in a wealthy/old money-ish neighborhood. The cafe had terrible tips despite there being made to order food and tons of regular customers. We just made fun of stingy customers behind their backs. No one I've ever known in food service of the bar industry HAS EVER tampered with or spit in a drink/food. First and foremost, you'd be fired on the spot if anyone saw it. Also, food service people, and this may surprise you, also eat in restaurants.