Good question.
Last winter was my first with a spa. I kept it set for F3 because I have the mindset that more is better. Then this past spring I experimented with less programmed cycles and there was no change to my water quality. I guess our 24 hr circ pumps do a pretty good job by themselves. So I have been keeping it on F1 ever since. I even noticed a decrease on my electric bill. I think the key is to set it for the lowest amount possibly w/o effecting your water quality.
I might set it back up to F2 or F3 if I start noticing that when I go out to soak, my water is constantly below my set temp.