My first one I did untrimmed, there wasn't a whole lot of fat left when it was done. This weekend I'm trying one trimmed and untrimmed, and see which I like better.
Untrimmed obviously you have less fat to pick out and pitch. I've seen some recipes that call for the fat cap to be added to the finishing sauce or mix-in sauce as it's cooked, then strained out, to add extra flavor. And when I trimmed my butt for this weekend, it lost it's compact shape and kind of spread out, so I assume the fat holds things together a bit as well.