How can I charge my iPhone without waking up my MacBook Pro?
It has to do with connection activity on the USB data lines in a usb cable. For example, I have a Matias Tune Blocker connected to my Mac Pro. When I have the switch set to sync my iPhone, aka its acting as a regular USB data cable for for a USB device, the Mac Pro will wake from sleep as soon as the iPhone is connected. 

However when I have the switch on the Tune Blocker set to charge only, eg only the USB power pins are enabled, the Mac Pro will not wake from sleep when the iPhone, etc is connected to the cable. So a solution to keep your mac a sleep when connecting your iPhone would be something like a Tune Blocker, or making a modified USB extension cable that cuts the Data wires for the device plugged into that extension cable.