Longer nipples only needed if the rim has some kind of deep eyelet/insert or epicly thick spoke bed or a very deep V that means not all the flat is showing and you can't get a spoke key on it.
I have a theory that longer nipples could build a stronger/stiffer wheel, as they could offer the spoke more support, but probably marginal differences even if it was true. I've built with 12-16mm and never noticed a difference in ride quality
Longer nipples only needed if the rim has some kind of deep eyelet/insert or epicly thick spoke bed or a very deep V that means not all the flat is showing and you can't get a spoke key on it.
Standard alloy rims you won't need them.