As most have mentioned, personal preference. Fat trim is not needed at all, however I do it for anumber of reasons. The fat does scrape off much easier once cooked, but to really get some rub onto a lot of the meats surface, I trim all the side fat from the flat (especially for comps, for presentation points!), the large hunk of fat between the point and flat, I'll cut that away as well, then separate the flat from the point about 3 to 4", so that I can get some rub in there as well. On that piece, I'll try and leave fat on the bottom of the flat portion, but trim fat off the top of the point area to expose that meat and allow rub to adhere along with more direct heat to completely cook the point. As far as the bottom of the brisket (the fat cap), I'll only trim the heavy areas, leaving a uniform layer to protect the meat from the heat coming from under the brisket as I cook fat side down. As far as the theory of cooking fat side up to allow the fat to melt into the meat, I do not beleive that fat will melt 'into' the meat. The fat content in the meat itself is what were trying to get to render and break down to allow the meat to become both tender and juicy. The outer layer of fat may self-baste the brisket as a whole, but I find it hard to beleive that it melts into. I have cooked briskets any which way you want, slow and low, hot and fast, fat up, fat down, injected, at room temp, right out of fridge, they all turn out the same, cook them until they are done and enjoy. Each one tends to cook differently, just have patience and let it cook until it's toothpick tnder, rest, slice, eat. Just my humble .02...