Before laying the insulation down, or before blowing it in, seal up any openings in the floor. Electrical wiring comes through holes, caulk them closed. If you are blowing in insulation or packing the batts in tight there, buy prop vents. These are styrofoam panels that fit between the rafters to keep a channel open. That is good just to save on your heating bill as well as help to eliminate ice dams.
Ventilation is important for many reasons.
Key among them is the elimination of moisture that finds its way up to the attic. Moisture that makes its way to your attic will condense in the cold air. It will condense on your plywood roof decking and cause it to rot, and it will pass up through the shingles causing them to fail years before they should. It also keeps the attic cool in summer, which is also good for the roof, and good for your comfort in summer. Yes you have all that insulation up there, but an attic that is 160 degrees will certainly make the rooms below the attic very warm indeed.