Basically there are 2 different kinds of turning. One to set your neck brass thickness to a certain measurement and the other to just clean the necks up like Forester said.
In a custom tight necked chamber you have to turn off enough to allow enough clearance, usually .003". In that case you have to turn brass off the entire neck
To just clean up the necks you set the cutter to just hit the high spots
IME turning brass off the entire neck is more productive in that it reduces runout more and the necks stay more consistant throughout their life. You should determine how much clearance you have in your chamber by measuring the outside diameter of your fired unsized necks. If you can keep your clearance between the loaded round neck and the chamber within a reasonable range then you can clean the whole neck up. If it is .006" or larger then cleaning them up is better to keep from working your neck brass any more than necessary.
I turn new cases before firing and monitor them afterwards. Usually that is enough.