The time of download of an IPSW doesn't matter. You can download the iOS's IPSW any time. You can download the iOS 6.1.2 IPSW now, and use it later, which is the exact same thing as downloading the iOS 6.1.2 IPSW when you get your device. It doesn't matter since iTunes does not look at when you downloaded the IPSW, it looks for the proper SHSH Blobs and APTickets. Even if you have the SHSH Blobs saved for iOS 6.1.2, you cannot perform a custom upgrade to iOS 6.1.2 once the signing has closed because there's no exploit to take advantage of to install custom IPSWs on A5+ devices