Relax into a natural posture

While we're talking about relaxing, let me throw out this little point: Resting on your heels is not the same thing as relaxing. It is actually just the opposite. While standing on your heels relaxes some of your leg muscles, allowing them to rest a little bit, it screws up your posture throughout the rest of your body and causes much more tension than it releases.  Standing on your heels is the lazy man's path to a poor substitute for relaxation.
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Try this: stand in shizentai (natural, upright posture, hips over balls of feet, nice vertical, relaxed posture). Now, without changing your upper body posture, rock back onto your heels. I bet you feel like you are teetering on the brink of falling backward. Now, relax into this posture with heels on the ground. How did you compensate for the backwards tendency? Probably with the butt out behind you, trunk leaning forward some, and head hanging in front of your torso (as in the photo above). Now, imagine a string pulling upward on the crown of your head, drawing you upward into shizentai. Your hips rocked back over your toes as you were drawn upward into a relaxed, upright posture.

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Standing on your heels places too much weight behind you, causing you to have to compensate by placing some weight forward of your center. The most common way for this to happen is to hang your head forward. While this seems like a relaxed posture, it places muscles throughout your torso and neck under tension.
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The moral of the story - get off of your heels and relax into shizentai!