Yes, of course it depends. I let a once-in-a-while thing slide. But you can tell, I think, when this position has become the way that they sit naturally. And that's when I will correct.
Ashleigh; I just printed out some info off the computer, with a graphic of what I mean by "W-sit". I ask for the parents co-operation in correcting, but that's about all you can do. The little girl that I referred to earlier never really did get any better. I just don't think mom and dad saw it as an issue.