If I inject, I normally do it the night before. BUT I think it is more about which meat I inject. I have injected fowl for more years than I care to admit too. But I am just now trying beef and pork. Fowl nearly always involves a butter fluid transfer where are pork and beef usually use a fruit juice or broth.
That said I see no reason for doing it ahead in fowl besides the comfort of having already done the day of smoking. Why? Most injections involve butter as the fluid carrier. If you use butter the night before it chills and clots and doesn't move.
The pork or beef is completely different, the juice or broth does not congeal, thereby allowing the juices to migrate through the meats muscle fibers carrying the flavors you are adding with it..
I also always massage the meat when injecting to prevent squirts or leakage, while being careful to use very small amounts injected in one place at a time to prevent pooling. Pooling is not good.