Recently, I've noticed that a lot of teachers (myself included) have said some version of the following to students: "It doesn't matter if you ever do "x". In this case "x" is some challenging pose or transition. Leg-behind-head is a popular one to insert into the sentence. What I mean is: "Don't obsess over poses. Yoga is more than poses."
Lately, though, I wonder if this is a disservice to students. If there is anything that "doesn't matter" then, why try? If we don't put forward any effort towards the places, poses, tansitions that are challenging, then half of what makes up yoga asana according to the yoga sutras is not there. The yoga sutras say "Sthira sukham asanam" Yoga asana is effort and ease.
So back to legs-behind-the-head...and the question: Does it matter? Full disclosure, yes I can put my leg behind my head and on most days I can put them both back there...but does it matter? On the one hand, no. Of course it doesn't. On the other hand, this has been one of the most challenging aspects of practice for me. The effort required, the attention needed, and the patience to practice these poses for years while seemingly no progress was made were transformative. In that sense it matters.
So, does it matter if you put your legs behind your head, come up to standing from backbend, or hold a handstand? No, of course not. It matters that you do what you can do with attention and breath. Does it matter if you try to do what challenges you in practice? Yes, I think it does. If there is no effort involved, then it is not yoga asana. The work to make the impossible become possible is where the yoga happens.