electricity moves at the speed of light... it doesn't care which terminal you unhook first..
and besides, i believe that it "flows" from negative to positive so your logic is backwards...
i always try to remove the negative cable first, if only for the aspect of accidentally hitting anything metal with the wrench while unhooking it.. once the negative is off, you can jump the positive straight to the body and nothing will happen. if you've ever tried to tighten a battery cable on a starter without first unhooking the battery (like i did when i was young, burnt a hole in a header tube on my Monte Carlo) you will understand this..