Hi Yonita,
Sorry I didn’t see this three years ago when you posted it
The experience of asana is one of constant movement toward the full expression of the pose. This is for the same reason one uses a garden hose without kinks in it to water their veggies - the water flows optimally in a structure that has the least obstructions.
In standing poses said movement is toward or into the plane of the pose. The question of alignment of the feet, as you’ve put it, appears to only apply to open pelvis poses, though I must admit I’m not at my sharpest and may have overlooked something in my thought process.
The optimal alignment (and I personally am using Light on Yoga as a barometer since Iyengar’s work on refining asana is unparalleled) is for the second toe of the front foot and the center of the front heel to bisect the back arch.
For those students who cannot “do” the pose that way because of blockage in the physical body or due to balance issues (also blockage, obviously) those students do heel to heel. Students who still cannot find stability there move the back foot even wider, not so that they will get the full effect but so that they too can “do” the pose and continue movement toward the full expression.
I’m certain there are other anatomical issues at play, specifically with the alignment of the pelvis (illiums), maintaining the hip in joint, and stability in the SI joint.
Hope this helps.
gordon