I'd like to say that everyone I have taught over the years including advanced students and teachers, needs to remember to look down a couple of times and check which direction their feet are pointing in, each time they practice. In moves where the feet are supposed to be parallel, a toe even slightly out here, a toe out there and I recall images in my mind of each student I've taught being reminded consistently to point their feet in the forward direction. The phrase "...feet are parallel..." is more than just talking about it. Now here are some of Master Hwa's practice reminders from the Classical Tai Chi Forum that mesh with his recent videos on keeping the knees and back healthy:

"I wish to reemphasize some of the points in the video to help you visualize and think about key elements of practice.
BODY POSITIONS:
In the instructional video one has to be concerned with many aspects of the position of the body. Two of primary importance are the (Master Hwa has changed this to "stretch the tailbone down" in his videos) tuck in and the knee over toe Tuck in or bringing the pelvis forward and up is easy to forget when you are thinking about all the other aspects of practicing tai chi. Yet it is of utmost importance. If done correctly then the practice of tai chi will alleviate lower back problems. If not done, then the movement can exacerbate lower back problems.

Knee over toe eventually can harm or injure the knee. Your step size can more or less determine whether your knee will go over the toe. The step size in the video with the front foot heel even with the back foot toe should normally avoid the problem of unconsciously bending the knee over the toe.
If you have a knee problem to start with,, you should try to use an even smaller step size. Keep the front foot heel just behind the back foot toe until your knee feels more comfortable. With such small step size you can till learn the form movements and develop the Internal Discipline."