i tend to totally cover mine - as it means i have more time between having to repeat the process
some people like to leave the very tips out, some cover them totally
the plants don't seem to mind!
i have a couple of rogue spuds from last year that i didnt clear out, growing in the middle of my beetroot (and i've just had one pop up in the onions too!)
i'm not gonna bother eathing them up