It depends. If you are jumping for peak power output, then you could step down to minimize wear and tear on the body, and allow yourself to recover between reps. If you are looking for endurance (or if you need to do multiple jumps in a row for time), then maybe jumping down is an option. Note that jumping down does increase the loading and stress on your body. So if you are doing high volume box jumps, stepping down may be a better option for most people.