You don't need dedicated mouse buttons, my current cab has them, but only because the previous owner of the game had already drilled out his config in the panel and it had them. I just generally hook up the start buttons to the mouse input. Mame is the only emulator that needs start buttons, and mame can map the mouse buttons as the start button just fine, while many other emulators can't map the mouse buttons, which is why I say to use them as the start buttons rather than just as a couple buttons on the player 1 buttons bank.
Also, if you cab is a completely menu driven cab and you have no need to navigate windows with the mouse then you can just simply not hook up the mouse buttons at all.
Also, if you are really going to all that trouble to build a pac-cab then shell out $25 for a Midway coin door.