I think it depends on the smoker. I did my last one down, it turned out rely good.
It absolutely depends on the smoker & heat source.
100% the only thing IMO that matters is the heat source location. If it's directly under the meat like a barrel smoker, vertical smoker, kamado joe, etc. the fat goes down. This keeps the fat between the heat and the meat which keeps the meat from drying out. On an offset, the hottest part of the grill is towards the top of the lid. So fat side up for the same reason.
At the end of the day, it depends on how picky you are. All the little crap you do makes your brisket a little better. Do none of the little stuff and you can have a really good brisket. Do all the little stuff right and it will be great. But how much work do you really want to do to make something really good just a little better? I'm talking injecting the meat, let it sit seasoned overnight, keep full airflow to the fire and don't choke it, adjust the wood every 20 minutes to keep the temperature where you want it,
don't use pellet grills, spritz the thin spots so the whole brisket cooks at the rate you want it to, buy specialty pepper and then age it, and all the other crap.