Ah, that's the million dollar question. As you might already know from this board, I'm a mum with Asperger syndrome who's had a formal diagnosis on more than one occasion (well, it keeps the specialists in work! )
All depends how much difficulty we're in. If she's coping well with friendships, school, eating, drinking, sleeping, health, habits, cheerfulness etc, then I'd say there's sometimes not a lot to be gained from a diagnosis. But if she's struggling with any combination of those things, it's worth getting it checked out.
Having been through life with everyone quite convinced I was 'coping just fine' and with me silently screaming with fear through most of it, I know what a relief it was for me to realise that there's a reason for all of the challenges I've had. It's helped me to be kind to myself, to adjust my environment to help me, to be more honest with people about the things I find hard, to know that I can ask for help (and sometimes there's someone to answer that plea)(not often enough for adults though). I don't think we grow out of it. We can get better at disguising the pain, fear, surprise, uncertainty and unpleasantness of living in an unadapted world with no formal help, yes. But that's not really the same thing as getting the right help to make our lives as easy as they are for most other people.
So, it's a personal decision, but my personal view is that it can be very useful indeed.
Have you seen the standard checklists for an ASD? You can google "AQ test" for example or the Cambridge "Autism Research Centre" and look for the tests bit there, which lists ones for children too rather than just for adults. It's not a diagnosis, but it's a clue as to whether a proper diagnosis might be a good plan?