DD2 turned 4 last week and spends most of her time on her toes - she's my little fairy. I've been concerned about it in the past because as you say, its an indicator of autism, but she's been assessed.

Also been concerned because of it causing potential issues with muscles and her growing skeleton but been assured by her paed that because she can walk flat footed, and does at times, its just her thing. We're keeping an eye on it, but not panicking that its something sinister. Definitely worth checking out though, if for nothing more than to put your mind at ease.