Well, actually...Sarah Powers sometimes says that the 4th principle of the practice is "Come out slowly." So, maybe yours is the 5th principle?
However, I am not sure I fully agree with your premise. I agree in general that when the bones feel support, the muscles can relax. So most of the time, propping up whatever is floating could be a good idea. But, for some students, this can go to far. For example: in Saddle pose, many students have floating knees. As they stay in the pose however, their quadriceps relax and lengthen allowing the knees to go lower. If the knees were supported too early, this release might not happen.
Maybe we are saying the same thing: let nothing float AFTER you have reached your final edge. The final edge is where no more relaxation is going to happen, thus no more movement will occur. (I am using relaxation here in the technical sense relating to creep of the connective tissues, not to a psychological relaxation.) But, you can also avoid floating if a stress is too intense and the student will not be able to linger long enough to get benefit. In these cases, no matter how much time has passed, propping may be a good idea right from the start.
Cheers
Bernie