For the past couple of decades, I’ve been taught and teach that you should time your cuts so that your foot lands at the same time your sword hits the target. And I continue to think that this is a good way to instruct novice fencers.
You don’t want to step before your cut, because that brings you into measure without a creditable threat. If you try, the observant opponent will strike you as your foot lands.
Conversely, you don’t want to step after your cut completes because that leaves your foot in the air. And obviously you can’t have a strong posture if you are balanced on one foot.
Currently I am studying Suiō-ryū, a living Japanese martial art. One if the things the instructor has started to insist on is improving the timing of our steps. Rather than having the foot land at the same time, he tells us the foot lands just before the cut. Roughly where the outline is in this illustration rather than the target itself.
The basic idea is that you want to stop your forward movement and form the stable posture before making contact. That way you can better deal with any recoil or counter-cut. You do this by landing on the ball of your foot.
This leaves some excess energy, your leftover momentum, which can be channeled directly into the sword. Which in turn means that you actually hit harder than if your foot lands at the same time. (Or at least that’s how they teach it. Measuring the absolute power of a cut is hard without scientific equipment.)
Grauenwolf's Study of Western Martial Arts 