Depends if there is a lot of metal debris in the ground and how long you have to hunt it.
If the ground is fairly clean and you have lots of time, dig everything above iron unless you like digging rusty iron objects (I don't).
If time is limited, set to just under nickel and dig all repeating targets (the ones that don't try to disc out as you repeat your swing over them) If you have TID, dig targets that don't radically change their ID as you sweep.
If its littered with targets and you have lots of time to keep going back, set your disc at zinc penny range and work the area. Then turn your disc down lower each time you return to rework the same location.
If not a lot of time, then set just above pull tab and dig the good signals only.
Basically you will want to make the best of the time you have. Most one way "and/or" iffy targets are trash. So if your time is limited, dig the signals that are fairly good.
Time and experience will tell you when to dig iffy targets. If you aren't sure where to set your disc, grab a target you wish to disc out (like a pull tab or zinc penny) and set your disc using that.
Hope that's helpful.