Alignment question of the day: “Should I keep my big toes together, or have the feet hip-width apart in Tadasana and similar poses?”
The simple answer is that it has a lot to do with skeletal structure. Those with narrower hips and a relatively vertical angle between the femur and tibia (and presumably also a short femoral neck and/or more open femoral neck angle) will find it relatively comfortable to keep the big toes together. Those with wider hips, knock-knees, or sharper femur neck and femur/tibia angles may need to work with the feet wider — at least for the initial stages of practice.
Related to the structural considerations, some people need to go hip-width for two primary reasons: stability or space. Some have difficulty finding stability with a narrow stance, and others need more space in the hips (not so much for Tadasana, but for the Uttanasana that follows/precedes in Surya Namaskara) — and usually some measure of both. Touching the big toes in Tadasana challenges this stability, which can be both an advantage and disadvantage, depending what stage of practice one is in and what the emphasis is.
Challenging stability and then managing that challenge is a great deal of what we do in yogāsana. However, introducing challenges to stability assumes that stability has already been found at a more fundamental level. A piece often missing about touching the big toes together is that this contact gives proprioceptive feedback against which to push when abducting the big toes — big toe abduction being a tremendously stabilizing action for the feet and lower legs.
Narrowing the stance in Tadasana by touching the big toes becomes more important when working toward one-legged balance poses: Vrksasana, Ardha Baddha Padmottanasana, Virabhadrasana III, Uttitha Hasta Padangusthasana, etc. When standing on one leg, the supporting foot is either on or close to the midline: if it were under the hip joint (or wider) you would topple over. So having the big toes touching in Tadasana prepares the vestibular system for the narrower and more medial base of support that will (presumably) come later.
So, structural considerations notwithstanding, emphasize touching the big toes together when working with or towards one-legged balancing, proprioception, and the inner ears. Otherwise, work with the feet hip-width apart when greater stability is required and the practice does not include any one-legged work, or when more space in the pelvic girdle is desired or necessary, such as with some lower back problems, hip/groin restrictions, hardness in the pelvic and abdominal cavities, and/or when pregnant.
The same goes for other poses that have discrepancies as to what is standard or so-called proper alignment. Remember that different alignments have different effects, and that one may or may not be more helpful or “correct” than another — these differences are contextual, not absolute. Remember too that structural considerations are only one of these contextual variables: sometimes the rest of the practice must be considered when making decisions about which alignment to use in more fundamental poses.