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

Never heard of that one. 

Just turn off the DAC first just to be safe

Why not just play it safe and do it, or you can also try frying your electronics but that seems unlikely. Could be for two reasons I’m familiar with, either just potential damage from the sudden electrical connection, which frankly seems doubtful, or the ability of the devices to talk to each other if the USB "hand off" needs to be done a certain way by either device.

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.