[JRTJH]

I think it depends more on the water quality than whether the tank was empty or might have had a "little bit left in it".... If your water is untreated well water of questionable content (organic matter, nitrates, etc) I'd probably flush it well (at a minimum) or re-sanitize if you're unsure. It probably is an extra, unnecessary step, but if you've ever gotten sick from "bad water" you'll "forever be cautious".....

If, on the other hand, you're using well chlorinated water from a known good source, you'll probably be OK just running a little through the plumbing to flush out the stale water, filling the tank and heading out.