This is more of a personal preference and it will be different for everyone. I always cut mine at least one inch on each side. My current carbon Monkey Lite SL's are down to 22.5 inches. Some will say that making the bars shorter will take a slightly stronger effort to turn. It may be, but down two inches from normal length, I don't notice the difference in effort. I do notice a MAJORr difference in confidence. No problems on the quadrants at Boyette now.
You can cut carbon bars just be careful on which saw you use. Plumber's pipe cuting tools are good. If you use a hacksaw, make sure the blade is brand new and very fine. If you take the bar off and bring it to a LBS, they might cut it for you for free.