I'm no expert on TIU addressing, however, if I only used one TIU at a time I would make them all the same address and that would be #1. Sure seems a lot simpler to me to so it that way.
I only have 1 TIU so I have never bothered to study the use of multiple TIUs or how all that works in Barry's books. If I get a back up TIU, which I am thinking about, it will be address #1 just like the one I have now so I can just swap them out if one needs repair.