Bones, the CHT in most 206's is a rather rudimentary gauge. If yours is any good, and assuming it is fitted to the hottest cylinder, I would aim to cruise with the CHT's always lower than 380.
So you could try cowl flaps shut under 370, approaching say 375, cowl flaps open or part open. You will need to experiment. Watch the oil temp as well but here I would just monitor the red line. Approaching oil temp red line, cowl flaps open regardless of CHT.
Unfortunately most CHT gauges are not that good. If it was my aeroplane it would have an engine monitoring system that looked at all cylinders.
Opening the cowl flaps after a long descent will do no harm whatsoever, my preference would be to open them on downwind or according to POH.
Closing them will give a few extra knots and may provide more uniform cooling but again without a monitoring system you really have no idea.