It all depends on where you want to draw the line. You don't have to insulate at all. However, your bees will burn more energy (food stores) over the winter to stay alive and possibly affect your yearly honey harvests and winter survival may be reduced (especially with top ventilation). Not much sense in forcing them to struggle like that.
You could go over the top and build ugly hives/cosies out of insulation board to be closer to that of a natural tree environment, it depends on whether aesthetics are important to you. I personally find this a bit over the top.
I prefer the middle ground, a thick slab of insulation board in the roof all year round. The hives are aesthetically pleasing and also moderately insulated.