I'd also like to point out that although it may be the owner's preference that there not be 7 buttons: What about the people that you invite over to use the machine? Maybe they WANT 7!
For example:
Let's say I prefer only 4 buttons for 6 button games. It may be easy for me to press 2 simultaneous buttons to make up for the lack of the 5th and 6th buttons. OK fine.
However, when a buddy comes over and says, "Hey, why are there only 4 buttons?" and you pipe out, "Aw it's no big deal, just press 2 at once to get the extra buttons. Most of the games I play only have 2-4 anyway, so you don't need them". They may be polite and say, "Oh, OK", but what they are really thinking is, "Man, that sucks! Oh well, beggars can't be choosers."
I don't know about you... but although I am building this cab for myself, the main reason I'm building it is for my friends that come over. And of course for "showoff value" - Hey why not? It took this long to build it right? The worst thing that can happen is that they are uncomfortable with the controls in my opinion. Why leave the job unfinished? Even if they only use it a few times, it's worth having that option. Because the one time they want to play something and you don't have the controls for it is that one time you wish you had put them in to begin with!
~ DeLuSioNaL