I'm not sure what this "theory of always unclipping the same foot" is, but I don't recommend it.

You should be able to unclip both feet. There are many situations, in real-life traffic, where you may need to unclip your "not-so-sweet" leg.

Practice both...
I concur.

I've never heard of unclipping one foot only as a theory of practice - I've only heard of it as a bad habit of people new to clipless pedal systems.

When I started using clipless, I had to consciously remind myself to practice unclipping my left foot, because I naturally was inclined to unclip the right. Well, naturally, I fell over on the left on a few occasions! I then moved on to unclipping BOTH feet every time I came to a stop sign, out of a fear or falling... and finally, I learned to gauge the situation, and unclip the correct side accordingly.

The good news? I rarely have to think about it anymore! Whew!

Don't worry! You'll figure it out!