I take a rather liberal view of what makes a good yoga practice. (Iyengar-style teachers and students might want to avert their eyes here.) I'm very much a 'sthira sukham asanam' kind of woman - that is, Asana is a steady, comfortable posture.* That's not to say I give my students an easy ride, as those who enjoyed/endured the Crow balance with me this week will testify; but I can't see that there is anything to be gained by insisting on perfect alignment.
Isn't it better to have one foot slightly forward of the other and to achieve a beautiful, stable Trikonasana, for example, than to struggle to keep the feet in the 'right' place at the expense of everything else? I hope it goes without saying that safety is always the prime consideration, so twisted hips and skewed knees are a no-no, but I really do believe that a little judicious tweaking here and there is a Good Thing.
Apart from anything else, when we 'achieve' a posture, whatever that means, it not only makes us feel good - and yes, I know: ego rears its ugly head here - but also boosts our confidence and makes us more willing and able to explore further.
I maintain that when working into a standing balance, it is much better to rest a tentative fingertip on the back of a chair and be stable than to try be in what is ostensibly the right shape but then to wobble all over the place. Two feet on the ground and arms overhead is a balance; one foot on the ground and the other limbs flailing around frantically is most definitely not.
* So says The Yoga Sutras of Patanjali, II, 46