This advice might come as a surprise from a yoga teacher, but if you approach movement with stretching in mind, you are missing a crucial distinction between what you're attempting and what is actually happening.
There's only one thing you need to concern yourself with; 'pandiculation'.
This is something you naturally do when waking up, and one that animals are seen to be doing all day (we should be too).
What is a pandiculation?
It's simple really, don't think of stretching, think of contracting.
Notice what is really happening when you yawn in your bed as you wake up. You are not stretching, but contracting.This is very different to stretching, which only serves to create more tension.
Question: What do you need, mobility or flexibility?
Answer: Mobility, as flexibility comes under the mobility umbrella.
You probably don't need anymore flexibility, but you may want it, especially if you're a yoga student/teacher.
Air types have the least amount of stability and should take particular caution when 'stretching'. Fire types may injure themselves more, but they have adequate strength for it not to be particularly dangerous: apart from the usual pulled muscles. Pitta types can create more problems over a longer period of time, just like an air type.
Water types need to be aware that they may have more protection, but only up to a point. At some point, they must be aware of moving beyond ranges of motion that an ordinary individual does not need to everyday life.
If you are a dancer, yes, you may need more of a range of motion, but most of us do not need it if we do not use it.
It's simple really; if you're weak, get strong, and if you're strong, get mobile. If you don't use it, you don't need it.
What earthly benefit can you derive from placing your legs behind your head?!
Mobility is not the same as flexibility, although mobility could be seen to encompass flexibility. In other words, you may be flexible and get your legs behind your head, but you may not even be able to lift one leg off the ground and make a circle with your knee, while standing on one leg in a controlled articulation.
Pulling legs into lotus is just one extreme example of what I mean. Flexible people can pull the legs into the posture, but those with more mobility would be able to do that without using their hands.
Please don't attempt to pull your legs into lotus pose. And while you're at it, please don't attempt to stretch, because that's not what is happening, anyway. Instead, lengthen your body, finding more space to move as you contract and release tense muscles.