I'm sure many of you heard the same thing from your web design 101 instructor: if you're styling an object that only appears once on a page, use ID. If it appears more than once, use class. Well, here's our version of this 'rule': If you're styling an object, use class. Period.
The reasoning behind this is simple. If you apply a unique style to a div on a page using ID and later want to apply the same style to a second div, what are your options? You must either copy the styles from the first div and paste them to a new, unique ID, or you need to change the ID of the first div to a class. If you would have declared the ID as a class in the first place, you wouldn't have to copy/paste or rewrite any CSS. You'd simply just apply the same class. And if you don't use the class for anything else, who cares? It won't cause any problems if a class is only used once on a page.