Good question, Mike, I’ve wondered the same thing myself. I’ve been harvesting from my Flow hives for two seasons (with great success) but I still struggle trying to decide whether to harvest the middle frames as soon as they’re full versus delaying the harvest to encourage the bees to fill the outer frames faster. I’m still not sure of the optimal approach. I’ll be curious to hear other people’s thoughts. I suspect it depends on a particular colony’s honey storage habits as well as the strength and duration of your nectar flow.
My first year I had a decent nectar flow most of the season and harvested each frame as soon as it was ready (I was eager) and although the end frames eventually filled up, I ended up harvesting from the middle frames multiple times before they got to that point. My second year I ended up with more partially filled frames, some of which I was able to harvest anyway because the honey inside the uncapped cells appeared to be cured (per the shake test), others I just left out for the bees to clean up before packing them away.
Further complicating matters for me is the fact that I have traditional supers stacked on top of my Flow supers to ensure the bees have enough space to prevent swarming and also to give them extra room to store upward if they want to (to discourage backfilling into the brood nest). I noticed that the bees would build out and fill the middle frames of this traditional super before finishing the outer frames of the Flow super. Since I hate harvesting traditional honey frames (Flow has me spoiled) I erred on the side of harvesting from the Flow frames as soon as they were ready to encourage the bees to continue storing new honey in the Flow rather than the traditional.
Like everything else to do with beekeeping, so many factors to consider and no clear answers!