Picture from: yogabeginnings.com
"Appropriate Poses" seem to be a theme lately in my reading and from the workshop I recently attended. One magazine article (sorry, I don't recall which one at the moment), placed a strong emphasis on not doing poses until you are ready for them. I understand the view point of the article - you don't want to be attempting poses beyond where you are currently working for safety and injury prevention.
And this is true. A beginner should not be attempting certain "advanced" poses -for example, inversions - until they have grown in body and balance awareness and strength. There are certain poses a person should not attempt if they have issues with injury without the assistance solid foundation poses. For example - neck and shoulder injuries.
This is a two part awareness, I feel. One, it is the blossoming awareness of the practitioner. To know what your body is capable of - that a stiff back and shoulders are not suddenly going to open up and permit you to do a full wheel (Urdhva Dhanurasana) overnight. That if you have problems with balance in standing poses, you are not going to be rock steady in headstand. It's the understanding that you may have to set your ego aside for a while and work on the basics of the postures. Ah! Therein is the yoga practice!
And the second part is the awareness of the instructor. They can see from the outside how you and the rest of the are moving. Over time they learn how your body moves, where you are tight and what you could use more practice with. When the instructor comes over and suggests you move back a level in the pose, they most likely saw a need for work on the basics. Yet, they may very well come over and invite you to move into the next level, seeing that your body has responded to something and it's time to start pushing boundaries a bit.
And that comes round to "appropriate poses". First, I completely agree for myself and my students, I/we shouldn't practice poses beyond our capabilities. I don't want to hurt myself and I certainly don't want a student to injure themselves.
However, how can I advance my practice and that of my classes, if I don't explore the next level? In the true traditional Ashtanga series, we would not practice beyond the primary series. In fact, most of us would only be practicing up to Marichyasana D and then we would all be done because we don't have the shoulder and hip flexibility to bind. But there are so many other postures out there that can assist in increasing flexibility and strength! You know, I've never done well with someone telling me "you can't do this because tradition says so".
I also believe by gently exploring the next level, you can learn what you need to work on to move into that next pose. Maybe it's shoulder strength. Maybe some more hip openers. Or core strength. You step back, work on a variety of poses and come back to that challenging asana at a later date and re-assess then. The important thing here is you RECOGNIZE you are not ready. You RECOGNIZE you need to work on some foundation poses first. And you RECOGNIZE your ego needs to be patted on the head and told "not today".
So I say, go forth and explore, but with awareness and mindfulness, and see where your practice will lead you.