Okay, hear me out for a minute. I know that in general, you aim to kick with the ball of your feet, the instep, or the shin. (At least with kicks like the roundhouse kick, which are traveling in that direction). But that's because we train in class barefoot, or in martial arts shoes that don't offer very much protection. But if you're going to be kicking with your regular shoes on (especially in self-defense), my theory is that kicking with the toe of your shoe is going to actually be the best option. It seems like it is going to be the safest part of your foot to kick with, since the toe is usually reinforced in front, and you're not flexing your ankle to do the kick (like you would for your instep). The toe of your shoe is also usually hardened, and is going to be one of the smallest points of contact to concentrate your force (the blade of your foot may be sharper, but it's also a longer surface, and it won't really work for a roundhouse. If you miss short, you might scrape them with your tread. If you miss long, you hit with your shin. Overall, it seems like with your shoes on, kicking with your toes would be the best way to kick. It's just a thought I had, and not something I've seen discussed before.