There is a good discussion on this very question currently running over on the Vegetable Gardening forum here. Leaves are only recommended if nothing else is available. And top trimming isn't necessary for most crops - optional.

Dave

http://forums2.gardenweb.com/discussions/3486281/protecting-root-crops-with-straw?n=5

I've only done it here in Michigan with parsnips and carrots. Grandma never let me dig parsnips until spring, they stayed in the ground to "get sweet" and only got dug in the spring. I think it gave a nice fresh vegetable when no others were available, perhaps.

Anyway, I didn't cut the tops, but I don't see that it would hurt to do so. We then made sure the tops of the parsnip root was completely covered with dirt, and added some more, then a layer of straw, maybe a foot thick. Grandma liked to cover the straw layer with a piece of plastic because then she could pull back the cover, remove the dry straw and dig her parsnips, it was easier than dealing with straw that had been rained/snowed upon, frozen to the ground, etc., plus it kept the straw from blowing off.

Annie

I did it for years (with hay, not leaves) with parsnips, carrots, beets and turnips--until the voles arrived. Now I can do it only with turnips, since the voles don't like them. And I can't grow parsnips at all since they really do have to be left in the ground.