Buying boots too big so you can "grow into them" will simply not work. You need them to fit your feet as is, no way around it. The only way to get rid of shin bang is to not ski backseat, and as far as boots go, the most you can do is get a stiff boot that can support a properly-balanced riding stance (which should at least be over the balls of your feet). If you like a softer boots, you at LEAST should buy a boot with a meaty shin strap so that the shin pressure is distributed primarily towards the top of the boot. (lower shin buckle holds the ankle, upper shin buckle/strap support applied force).
The problem with soft boots is that they can be deceptive. They may feel like they give less bang when you're riding up the lift, because there is less pressure applied, but the lack of support will definitely make it harder to ride with the basic athletic stance, thus causing you to stand on your heels and get more shin bang. As a boot fitter, I'm telling you that its retarded to buy boots without getting them personally fitted. Take your ski socks to a respected shop and Go! diddymow!