Thoughts of the Day

Suddenly I wondered, Is there any particular rule as to when you're supposed to use "!?" as opposed to "?!", and is it even considered proper grammar at all? Maybe I'm supposed to stick with "!!" and "??" even if they often don't quite convene exactly the emotion I'm trying to express. Or something.

...There may have been more to this question, but if there was, I've forgotten it.