The only rule I follow is to make as little as possible public.
Look at it this way. You can always make something public later on - it won't break any existing code. Trying to make something private that was public could well end up breaking a lot of existing code.
If someone wants more functionality from your class then they can make the request and you can expose what they need. Chances are that they'll want something that's subtly different to what you already have anyway so you'll need a new interface.
