I love this advice from East Side Bride. We're really trying to keep the guest list small for intimacy, money and space's sake. This is exactly how I feel about the matter.
"Re: plus ones. If someone is in a serious relationship, it's only fair to invite the significant other. Otherwise, you can wait until someone asks you to allow her a guest. But why make the singletons feel like [jerks]? You want everyone to have a good time."