I'd avoid turning necks if there is not a specific reason for doing so.
The important thing is to run the expander plug through so the mandrel on the turner has a good fit. I have a 243AI with a tight neck, so firing before turning wasn't even an option. If you know your chamber neck diameter, subtract bullet diameter and desired clearance. Divide the result by 2 and that's your desired neck thickness.
Turning virgin brass and cutting just into the shoulder worked well. I did, however, after several firings, run the expander plug through and turn the necks again just at the base to remove a bit of brass which moved up as the case changed shape to fit the chamber during the AI'ing process.
If I had the option of firing the brass prior to turning that's not a bad idea. It would get the case closer to actual chamber dimensions before removing material and make it less likely to need a touch-up job later on.