Which is more profitable. Paying $0.08 per kWh and running 24/7, or paying $0.06 per kwh and only running 18 hours per day (and powered off the remaining 6 hours per day)?

With a low margin environment like we have today, powering off for 25% of each day would actually be more profitable, if that were needed to obtain the discounted rate.

So no, ... it's not an absolute that a miner must run 24/7 to maximize profit. There are other factors that come into play.

Energy storage, especially with today's electric battery technology, is incredibly inefficient, nonetheless extremely capital intensive. At least for something like supporting bitcoin mining.

Electric battery storage certainly can be justified in many instances, but bitcoin mining is not one of those.