In his book On Food and Cooking, food scientist Harold McGee writes, "A metal spoon can induce crystallization by conducting heat away from local areas of the syrup, cooling them and so leaving them supersaturated [causing crystallization]." He recommends using a wooden spoon, which doesn't conduct heat.
Our chef's justification for using metal spoons, which has always made sense to us, is that they are less likely to carry trace amounts of fat, air bubbles, or other food particles than a rough-surfaced wooden spoon. These foreign particles can also cause the sugar to crystallize.
So who do we trust: the Mighty McGee or our baking-veteran chef? What do you say?