Insulating my roof, do i cover tile vents?

No, no, attics need to be cold so the underside of the roof stays cold. The vents allow the roof underside to breathe. If you clog the vents, the underside gets warm and warm and cold air condense into water.......underside mold, upper side ice dams. Think of the insuation as keeping warm and cold air separate. Keeping the warm air in the house section.

The vents have nothing to do with temperature but humidity. If the insulation breathes you can cover the vents. If it doesn't you can't.

The point of insulating is to trap air to discourage transfer of heat or cold from the exterior of the house to the interior, and vice versa. If you insulate a space but then ventilate it directly to the outside, you are defeating the purpose of the insulation. If you insulate your attic by putting insulation between the rafters, you should change the ventilation system in the attic from roof venting. Add the attic space to the overall heating, ventilation and air conditioning (HVAC) system(s) that serve the rest of your home. Otherwise it's like insulating your house and then leaving a few windows open If you do not plan to heat and cool your attic using your homes HVAC system, then the insulation should be on the floor of the attic, not up in the rafters. Feel free to email me if you have questions