I would say underneath, because the purpose of it is to prevent weeds and also keep soil from drying out...if the straw is always wet it could start to mold underneath.....in late fall I cover my little garden with straw, then cover it with cardboard. I put peices of cut wood on top to keep it from blowing away....in the spring the straw is almost broke down and the ground full of happy working worms...I just mix the straw that's left into the soil to compost.
I would say underneath, because the purpose of it is to prevent weeds and also keep soil from drying out...if the straw is always wet it could start to mold underneath.....in late fall I cover my little garden with straw, then cover it with cardboard. I put peices of cut wood on top to keep it from blowing away....in the spring the straw is almost broke down and the ground full of happy working worms...I just mix the straw that's left into the soil to compost.