[pie314isi]

I suspect it doesn't matter. if the phone is powered and on, it's still doing the iCloud stuff in the background, even if it's locked.

[kevincox]

Note that there is a difference between regular "locked" and the "locked" after a reboot. For the later most of the disk is still encrypted so very little data and few services are operational. You can likely recieve text messages but I doubt much of iCloud syncing works.