I had this debate with some friends who have older children. We keep our liquor in the cupboard above the fridge. When they get a bit older (still just 8 and 10) and could potentially be tempted about drinking or have friends who might push them, we thought we'd probably get a locking liquor cabinet of some kind.
But then I remember lots of kids who had no trouble getting into their parent's locked cabinets, either having found the key or picked the lock. They used tricks like replacing the volume of vodka removed with water, etc. So the other side of the argument is to just leave the liquor in the cupboard and be more serious about educating your kids on the dangers of underage drinking, and create kids you can trust. In the end you have to trust them anyway, and if they really want to drink, they will find a way. Plus, locking it up just says, "I don't trust you".
So I don't know where I come down on this issue...