Even though you plan on a FOWLR, you may at some later time decide to put a few corals in there, or at the very least you're going to have some CUC (clean-up crew) like snails & crabs. For this reason, I'd advise on still using RODI water instead of de-chlor tap water..... there can be some pretty nasty things in your water that your FW fish can put up with, but corals and SW inverts cannot, namely heavy metals like copper.
Even if you start with tap and then switch over to RODI, you have still introduced these metals & chemicals into the system, and once there, they tend to accumulate in the rock & substrate. Almost all corals, and many inverts, are extremely intolerant of even trace amounts of copper, which you do have in your water from the pipes.
