At least if you're using the Ec2 optimized amis, Ec2 instances frequently boot fast enough that they'll be executing your username initialization before you get

At least if you're using the Ec2 optimized amis, Ec2 instances frequently boot fast enough that they'll be executing your username initialization before you get the run instances response. Though there's a long tail, so sometimes there can be a gap on the order of a couple second between the sync response and when the bootloader transfers over to the kernel.