Not a good idea. I talked to an older guy in my powerlifing club about the movment. Alot of lifters have blown out their lower backs on deads from a block. Including me. Thats when I started listening experience over theory. Three months of no squats and deads (no spinal loading) to recover. Endless sets of highreps pullthrought and goodmornings with the bar. With deads from a box, it's almost impossible to not round your lower back on them. And dont be stupid and tell me "this doesn't apply to you". Basicly if you deadlift conventional, you should have no problem getting the bar off the floor since lockout strenght is the sticking point for most lifters. Preform alot of heavy rack lockouts and goodmornings to help this. For sumo deadlifters, getting the bar of the floor is the hard part for alot of lifters. In this case, box squats are nice since it takes the streth reflex or whatever it's called out of the equation and gives you flexing strenght from the bottom possition. Theres alot more you can do. Just deadlifting with som supplimental lifts to help their sticking point works like a charm for alot of guys.