When trail riding, you need to be more comfortable, and if you are mostly just walking for a long time, you'll have to find a comfy position. unless you are posting, or jumping, you don't have to be overly worried about ankle down. as long as you are not pointing your toe down so that you have your foot going too far into the stirrup, or, in the case of long stirrups, losing your stirrup with the downward pointing toe. one can also end up with an upward gripping calf in such a position.
really, if your feet are more or less parallel to the ground, that is plenty. overly focussing on heels down often makes people push INTO the stirrup and brace the knee, which makes them pop up and out of the saddle if the horse should stop suddenly.