Roundhouse has different deliveries and foot shapes that depend on the target and or situation. Laurie and I train at the same school these days and we apply the ball of the foot as she describes in most situations except sparring-as we train Olympic style (we also use a slightly different path of delivery in some cases). However,for beginners-in general, when teaching kicking for the first time, we do not discuss "ball of the foot" until they have completed the first one or two belt levels as our emphasis is on teaching them the gross motor movements involved in kicking, i.e., chamber ( or knee lift), extension, and recoil. Transition to the "basic" foot shape (ball of the foot) is much easier when they are already fluid in their kicking movements. The same applies to front kick. Pulling the toes back properly takes concentration, flexibility of the ankle joint and toes, and time!