Hate to be vague but each setting has its own use. I like the highest frequency for di and si. Did you just pick the units up or wanting to get better use out of them?
I think its personal preference on what you personally want to see out of it and being able to distinguish what you are actually seeing. I do think too that it varies a lot between units and conditions. I'm constantly playing with my settings depending on what i'm focusing on. My settings will not necessarily be the settings that are best for you and your unit. Best advice i can give, adjust as you go and find what works for you.
Even The Sonar Doc leaves a lot on automatic. I like to turn up the scroll/ping speed a bit and turn up the auto-sensitivity a bit, and I usually turn surface clarity to low.