Join Date: Jul 2010
Location: East Central Illinois
Toes out are a symptom. The most important part of your seat is an upright torso, and weighted legs. Practice riding without stirrups for the first 1/2-hour to an hour. This will connect you with your horse's movements and wear out your ability to push yourself out of the saddle with your stirrups. It will also place your feet with your toes pointed mostly forward.
When you pick up the stirrups after this you will have a weighted heel.
No one tells you to grip the horse with your legs, but you must have a relaxed grip to keep you on the horse, or else you will slide off of him.
Deliberately pointing toes out is for ballet. Just as in dance, this rotates out the top of your legs, as well. It's great for balance and plies on the ground, but it causes you to lose your balance while riding.
Bear in mind that some people are not able to point their toes straight forward while riding. Therefore, some riders with very good seats look like they deliberately point their toes outwards. Perhaps your instructor is one of these people and has made the assumption that is normal and expected.