Never. If I bake a pie, I know it's going to spill so I watch for it. I open the door, bury the spill with salt, and keep on baking. When the oven cools I sweep it out. It sometimes occurs to me that I should put a cookoie sheet under the pie to catch the spills, but it's actually easier to sweep out the oven than to scrub baked filling off a cookie sheet.