My pyboards (v1.1) are powered externally via V+. I know from experience it is ok to hot plug/unplug the USB connection while the board is powered on (or at least I haven't had any issues in over 100's of times). However, some time ago I had an issue that made me suspect "cold" plugging it, i.e., having it plugged in when V+ is turned on, was not a good idea, so I haven't done it since...at least not if I remember. But occassionally I have forgotten and it seems to survive ok.
So I thought I'd pose the question to those that have more knowledge of the inner workings of the board. Is is ok to have USB plugged in before applying external power?