I don't take all the fat off to eat brisket at home, but for comps, I remove it. When I judged the one comp I've judged, a couple of weeks ago, none of the entries my table saw had the fat trimmed, and the judges all commented on that. None were trimmed to fit the box. With some, that wasn't a problem, but some were just too big for the boxes, so they looked bad.
I'm going to try trimming down a bit more, to ensure more bark all around. We include cut up pieces of bark in the box, but not underneath; I like the look of it in the box, too, and I want the judges to see it so they'll try it instead of not knowing it's underneath.