I would personally not wire any amount of current to a USB peripheral connected to my MacBook.
Switching a wire between mid-air and ground passes 0 current in both states which sounds like a pretty safe amount, though.
Its like grounding it, same thing you do when you touch it. pretty safe no?
I guess, but it depends on what the other end of the wire is connected to.
I guess a ~10kohm resistor in series couldn't hurt, should something go wrong. After all your finger is not exactly super conductive in the first place...