I leave mine uncovered, I think there will be less pests this way, I might be wrong tho. I'm not sure how much nutrient gets washed away, homemade compost is good as it releases nutrients as plants need them, so I dont think you lose much over winter. I've only topped my garlic bed so far, but I'll be pulling a bunch of veggies out by mid month so will add more then. I need to go to the beach to get some seaweed for the brassicas, I'll use that as a mulch too