I too was wondering if we should say hello you can read about it here:
https://www.cougarboard.com/board/message.html?id=28639173
But was with my daughter who just received a mission call a few weeks ago and she recognized him first and then I knew I had to say something. Glad we did.
I guess it depends on the celebrity...but I've met quite a few professionally over the years so I'm more apt to let them be if I'm by myself.