Hi!
I often get asked the same question by a lot of students and the answer is whatever suits you best, and what is expected in the exam. To be honest there is no correct/wrong way to do this.
Generally when I was dancing it I placed the heel down sometimes and sometimes I did not. On some level it depended on the music. It will also depend on you, and which ever method enables you to gain a nice swaying action.
It is important to bear in mind that in the Foxtrot we are looking for a nice zigzag motion across the floor, so this has to be thought of as well. What I will say though is you will feel the rise and fall in the Foxtrot as quicker than the Waltz for example. You start of heel-toe-toe and before you know it you have lowered and completed the fall, ready to begin again. Unlike the Waltz the Foxtrot does not require the feet to be brought together and then lowered.
So in answer to your question, I would say it is up to you. For me I go heel-toe-toe-and lower ready to begin a natural turn or whatever.
Hope this helps!