You might be opening up a can of worms here, as there's some dispute between competitive dancers and "book" dancers over this topic.
When I took my exams, I had to learn to lower far later than how I had been taught by all of my competitive coaches. Three separate examiners advised me that I should not begin my lowering until the body has completely reached the foot. However, I would never dance it that way in reality. Competitive dancers essentially relax the knee and ankle of the arriving leg, such that the lowering is complete once the body has reached the foot.
Also, you should realize that rise & fall appears different on passing steps than it does on closing actions. As your legs part, your body naturally lowers as a result, even if your legs remain equally straight (or bent) all the while. So whether you describe the lowering as beginning as the legs begin to part, or once the weight begins to transfer to the new foot, depends entirely on whether you factor the natural lowering into the equation.
Regards,
Jonathan Atkinson
www.ballroomdancers.com