3rd Mouse Button Function

Hey everybody. I got MAME running good and some good games are running. Thanks for your help. I am building a cabinet now and trying to decide about control panel button layouts.

My question is: Is there a lot of games that use the 3rd button (middle one) on the mouse. I am trying to decide if I should only have 2 buttons, or all three.

Thanks.

Does it really matter? As far as I know you can assign any key to any buttun while in a game and pressing tab... So 2 is good.