Depends on which one I am pulling and how strong the arms are. Small 4 arms I generally just have a hook in the trailer. For a 6 arm that is double hung, all the outside baits and the trailers get hooks.
Unless we are trolling in MD we put a hook in all baits. Usually the back bait is the only one to get hit but why risk having another bait get hit with no hook. Yes there is the chance a school could come through and take every bait but is that a bad thing? I have seen some pretty big fish all hooked to the same rod and all landed successfully.
I use a single hook on the smaller rigs, 4 teasers & 1 trailer. I use two hooks on the big rigs, 7 teasers & 2 trailers. On the larger rigs I hook the two trailers to a couple of the outside arms & run the parachutes 12" back from the arm. That setup consistantly produces the largest fish.