Is it worth adding a rest day in Moshi or Arusha before the climb starts, or is that a wasted day?
Should I take a rest day in Tanzania before climbing Kilimanjaro?
Yes. Plan at least one full day and night in Moshi or Arusha between landing and the first day on the mountain, and two if you fly in from the Americas, East Asia or Australia. That day sorts out four things at once: the long journey, the time difference, the heat, and any luggage that did not arrive with you.
The full answer, kept up to date: Is it wise to have a rest day after I land?