The vast majority of food service workers won't mess with your food because they're decent people who are used to the abuse and slander (or libel in this case) that customers throw at them. There are other food service workers who won't do it because they care about food safety. There are still others who won't mess with your food because they'll be fired if they're caught doing it. These three groups will talk shit about you if they think you're out of earshot.

Then there are the rare cases where the person is too vindictive and too short-sighted and might mess with your food (but will likely be caught by someone who will fix or remake it before it gets to you). These people never last long in the industry as the rest of us hate them more than customers and will drive them out.