Tomatoes are, of course, safe to use in cocktails when handled properly. Tomato vines, leaves, and unripe fruit, however, have proven poisonous to farm animals eating these plants.
However poisoning in humans by consuming tomato leaves (in non-extreme quantities/concentrations) is probably not much to worry about according to a story in the New York Times [link].
The story cites the following research:
According to “Toxic Plants of North America” (Iowa State University Press, 2001), by George E. Burrows and Ronald J. Tyrl, a toxic dose of tomatine for an adult human would appear to require at least a pound of tomato leaves. These authors conclude that “the hazard in most situations is low.”
Some people are allergic to tomato leaves but not the fruit. If tomato leaves are used, consider listing this explicitly instead of just "tomato" on the menu.
"The Tomato, Accused, Yes, but Probably Not a Killer" New York Times [link]