Zombies tend to either be good at either fighting or getting tips. Certain zombies are simply neutral and are not good or bad at either.
It's best to hire zombies that are good at one thing (like serving and getting tips) and hire some that are good at the other (fighting).
Remember that the better a zombie is at getting tips, the more money is acquired. As such, infect zombies like politicians, executives, and celebrities as waiters and waitresses. You should typically have more serving zombies than fighting zombies, since serving zombies are in constant use.
But remember, a zombie with more energy is able to serve longer so if you choose to turn off "auto-lock" and leave your device free-serving then it is ideal to have zombies with more energy serving.
On the other hand, infect zombies like engineers, foremen, and boxers in order to quickly kill off any competitor restaurants in the area.