Ok if you are cooking over a high direct heat, it makes sense to go fat down to protect the meat from the chance of charring.
That said, I doubt that the fat can render itself upon the rest of the meat when cooking fat up.
I say cook it the way that works for you and be happy with it. It you are cooking your meat and it dries out, then you need to take it out of the smoker sooner. If it isn't getting cooked thoroughly leave it in the smoker longer. Neither of these two methods will be dependant upon whether you cook fat up or fat down.
Just my 2 cents!
"Grilla in the Pits" "IMAGRILLA"
There's plenty of room for all of God's creatures.
Right next to the mashed potatoes.
Brinkmann Model 6668 Gasser
Stumps GF223 w/ Stoker assist
ECB that wins the $
Weber One-Touch Gold 22-1/2"
Large BGE w/ Stoker assist
55 gallon Drum Smoker
Wood Crate Cold smoker w/ smoke injector
No need for certification, I already know good BBQ!