Fine hair needs to be conditioned with every wash. Over-conditioning is actually very difficult to do. You would have to be using a super heavy conditioner and not rinsing it out all the way at least, oh, 10 times a day and never clarifying for that to happen.

Personally, I have very fine, thin hair and use at least 3 different conditioners every day. Of course, I clarify once a week. But even when I didn't clarify at all, I never had a prob with over-conditioning.

But to better answer your question, yes, it's perfectly safe to do COW's (conditioner only washes) on fine hair. I've certainly had to do this once or twice when pressed for time!