This sort of conditioning practice has been discussed on MAP many times and so far as I can remember it's always been considered nonsense. I'm neither an expert or a kickboxer. But consider the damage people who break blocks with their fists cause to themselves? This is basically what you're doing to your shins soft tissues if you do this sort of thing. Eastern martial arts are chock full of bad practices that prevail because it's the traditional way to train. That doesn't make it the correct way to train. Personally I'd focus on heavy bag work to build up the bone density in your shins. That will serve you far better in the longer term than killing off the nerves in your legs.