No problem. I used a tool like this:
http://www.buy.com/prod/prima-tools-.../90140328.html
Which often times is what they use at the LBS. Then used some metallic sandpaper to smooth out the cut a little so as to not slice up my oury grips instantly. Also, cutting while the bars are mounted on the bike may be even easier for leverage, if you don't have a vice or something.
I have heard that, yes, there's no real point in making them much smaller than your shoulders, in terms of comfort and getting through traffic. It's not going to help if they're only 3 inches on each side if you're trying to get between a cab and a parked car, because of your shoulder width.
I'd try riding around for a few days and see where your hands feel most natural/comfortable before making the chop.