If all you are cooking is the flat, I would definitely inject. Injecting isn't really all that needed for a packer, IME, but is for a flat. I don't really do anything special when I inject, just beef stock. Add a simple rub of equal parts salt, pepper, and mustard powder (or whatever your rub of choice happens to be) and you are golden. If you are injecting and adding a rub, look out for double salt jeopardy.
22" OTG; 22" Performer; mini-WSM; Brinkmann gasser; UDS (on the way); Maverick ET-732