Well if your using a I-pac it doesn't matter too much, since where ever you put the wires corresponds to a keyboard button. Which in MAME can be assigned to any button.
I'd just wire 1p side on one side of the i-pac and 2p on the other. Also have 1p connected to one ground, and 2p to the other so when things mess up its easier to narrow down issues.
If you were using a console interface like the Toodles Chutlu for a console arcade then the wires have to go in the right spots. In that case follow the manual it comes with.