It's probably up to your personal preference. I'm building a cab with a 4:3 LCD in it. Old games didn't use 16:9 (or 16:10), and so you'll end up loosing some screen space. The appeal to me of an LCD, was it's ability to easily rotate, therefore allowing games to be played using the maximum number of pixels possible. You actually end up with a bigger viewing area if you go 4:3 for the classics. Other's have used widescreen format LCDs, and are either OK with the waisted black space, or are even talking about hacking custom bezel arwork around them. That's your call. As for the newer games, maybe there's a good debate on why you should go 16:9 for them.. I'm not the one to ask about all that however.
Hope that helps.
-csa