So the reason for that is simple. The more leaf on the clone, and the more light that is supplied to it, the more it will try and photosynthesize instead of rooting. The idea that I (and most) use with cloning is less light and less leaf speeds the process. If you have less leaf the plant tells itself it needs to grow more. Sends hormones down but finds there are no roots to pull energy from. And instead will begin to root.
Whereas a plant with a lot of leaf and a lot of light will try and convert that light into energy for a few days, then revert to panic mode and begin producing roots.
I've done it both ways and really I dont bother trimming them down. If it's a top shoot, I leave the growth center and a fan below it. And lowers are easy because they arent in the same grow mode as an apical growth