From my experience I have had good luck trimming fat and good luck when I have not. So, I do not believe one way is any better than the other. For me it just depends on the circumstances.
I do believe there is enough fat inside the brisket so it shouldn’t dry out if trimmed. I do not believe the fat cap provides any self-basting benefit. So that does not figure in the equation either way.
The pros for trimming:
• I don’t like to trim away fat while the brisket is hot. It usually is slipperier and messier (even though it is easier to cut away). When it gets to that point, I usually just want to let the meat rest, slice it, and serve.
• It gives more surface area for the rub and the smoke to reach.
Pros for not trimming:
• It takes me a long time to trim, and I don’t want to mess with it.
• It takes me a long time and I don’t want to mess with it.
• I don’t want to mess with it.
If I do not trim, I will smoke it fat side down, so more meat will get the smoke. If I were to cook it fat side up, it might as well be in the oven and not the smoker (and that has been known to work in a pinch). The bottom line is, it all boils down to how I am feeling when I get started. If I have time and energy I will trim. If not, no problem!