Basic rule of thumb when dancing ballroom or latin: when you want to travel yourself down the floor - ie cover lots of ground - you step on a heel as you get more "drive". If you are taking a side step/chassis step/general small step or, as in the case of a rumba forward walk, a checked step (where you step but your body weight never quite settles over the front foot before transferring to the back foot again), you step on the ball of the foot.
There are some steps that this doesn't apply to, but it's the general rule of thumb.
I've probably confused you more though!