I believe that one must always have ones weight forward so as to provide proper form and connection in latin dancing. You may place your heel down in the backward rock steps but little or no weight is placed on the heel. To allow for the cuban action, you must shift weight from foot to foot, but 'most' of it must remain on the ball of your foot.
In the faster cha-cha for example, you simply will look too "clunky" if you place your heel down in the rock steps, and you simply won't have the time - not to mention have poor form.
I suggest you look at yourself in a mirror while trying to place your full weight on the heel and then negligible weight on it. You should notice the difference in your form even without a partner. Next try it to some cha-cha music, and notice how you will have to scurry through the 4&1 steps in order to make up for the time lost swinging your body weight all the way back and forward.
My suggestion - stay forward, and don't transfer much weight to your heel. BUT, don't dance on your toes all the time either, your calves will tense up and bother you before long. The object is to place just enough weight on the heel without 'falling backwards". Good luck.
sRi.