It's common sense kitchen wisdom that one should never garnish food with something that cannot be eaten, either because it is poisonous or because it simply tastes nasty. The idea is to just leave it off the plate and any unfortunate accidents will automatically be avoided. This is especially true if there are children around.
In the case of holly, the berries are mildly toxic and will probably make your guests sick if eaten. While it is rare for them to cause death, you really do not want holly anywhere near your food. The leaves are loaded with caffeine and while they are not necessarily toxic, they are spiny and unpleasant to eat. So leave the holly on the front door or mantlepiece and keep it off the dining room table!
Fun fact: Did you know that the well-known and loved beverage Yerba Mate is made from a spices of holly?
Related: Easy Garnishes for Simple Soups
(Image: Christmas Holly)