There's a couple schools of thought on this issue. You can approach it two ways:
1: Leave an airspace between the roof deck and the insulation. Ventilate the soffit up to the ridge. This will allow air to circulate under the roof deck and carry away any moist air that rises up from the living space to prevent severe condensation under the roof deck in cold weather.
2: Leave NO airspace between the roofdeck and insulation. Then prevent airflow from the living space to the underside of the roof deck. One common way this is done is with cellulose. (Resists airflow MUCH better than fiberglass) It must not be installed conventionally, but in a much tighter pack. This is referred to as "Dense Pack" Often 3-4 lbs/cu foot density. http://www.karg.com/pdf/Presentation...Insulation.pdf
I spent some time working on this same question. My house has 2X4 rafters and thus not much of a cavity. Certainly not enough for a vented soffit setup. There's REALLY tight spots now, so furring and a loss of headroom were NOT options. The house was insulated for 40+ years with Rockwool, and there was no rotting on the roof deck over the cathedral ceiling area.
This is what I know about Venting/not venting. Hope it helps a little!