My DS does it. He'll be 5 in July. He started doing it from the day he started walking and he only does it on occasion.

I know it's a sign of POSSIBLE ASD (and you need quite a few other signs for that to even be a possiblity, not JUST toe walking )... but that in itself is nothing to be freaked out about. Don't stress!
I know lots of kids who have done it.