It's common in a yoga class to hear foot alignment cues:
"Big toe mounds touching heals apart" "Outer edge of foot lined up with outer edge of mat" "Feet parallel" "Dig your pinky toenail into ground".
The list could go on.
My advice is to take all of these cues with a grain on salt.
These cues for the most part are bio-mechanically outdated and purely aesthetic. This is of course my opinion so take it with a grain of salt as well.
My teacher Tom Myers says that the feet should point in direction of knees (which usually isn't a straight line). I think this is a good place to start.
Try this with yourself or with students: Stand at top of mat with feet parallel and hip distance. Bend knees and jump up and down a half dozen times. Let you feet do what they want. Does one turn out a bit? Or in? Are they pronated or supinated? Take a mental note of this. How your feet land after these little jumps is probably similar to how your feet hit ground in walking.
Now ideally your feet would be pretty much parallel when walking but for most people this is not the case. They likely have one turned out a bit and may over pronate or supinate. I believe over supination to be more of a problem but more on this later.
Since how you walk and stand has a great deal to do with the alignment of your bones I think its important to consider. If someone has been walking with a turned out right foot for 30 years its going to greatly affect the myofascial layout of the knee and hip.
So how does this apply to asana? My findings are that where the foot goes and hows it's turned (usually out a bit) and if to pronate a bit or supinated some really depends on the individual. Do they have internally rotated femurs but externally rotated tibia/fibula? Do they have external femurs but tibia/fibula is more external in relation to femur and the foot even more external?
My thinking is that bringing the hip joint in to better function is a precursor to bringing the foot into better alignment. Since the hip is a bigger and more powerful joint I think this is a good strategy. If they have been walking with a turned out foot for some time and the teacher requests aesthetically pleasing feet I don't think its helpful. The foot needs to communicate with hip and if its aligned in an unnatural way (for that body) then communication is more difficult.
Try allowing your and your students' feet to do their thing. If the front foot wants to turn out a bit in a Warrior let it. Notice if you feel better transmission through hip. If feet want to turnout in bridge or wheel let it go and see what happens. Maybe the feet like to be a little apart in Tadasana or Chair and maybe one turns out a bit.
After sometime you might slowly start to bring the feet into more parallel but I think correcting alignment from the bottom up is kinda like putting the cart before the horse. I'm not saying everyone should walk like a duck either so don't get carried away.
If it makes sense to you try it out and let me know what you think!