Connecting laptop to dac via usb- gear on or off?

I certainly only make the connection with gear off. I was just curious if anyone had ever explicitly been told to do so (as I was by the dac manufacturer) 

Yes, I have a Mojo Audio DAC and was told by them to keep the DAC on except when changing/hooking up cables

USB standard allows for hot-swapping.  There are 4 wires in the USB cable/connector - 2 for power (+5V, GND) and 2 for differential signal (D+, D-).  If your DAC is self powered than it will be better to use 2 wire USB cable with signal wires only (to reduce electrical noise).  In any case +5V is short circuit protected.

Just hot swapped my Tab A  to my DAC, both on... DAC recognized Tablet audio content immediately and started playback.  Also said "usb" disconnected " when unplugged....  no I'll effects.

BUT if your owners manual says do not hot swap its probably good advice.