Of Corse there is always risk in tying off the stern.. If you have to do it increase your scope (length of anchor line) It should always be at least 4 to 5 times or more the depth of the water. A very short scope will not let the boat ride up and over a rouge wave.. It would be rare to set an anchor right below a boat.. But I have seen on light an variable wind days people pull the anchor rope snug right below the boat when the wind changes direction.. Thus not having any give when a rouge wave comes by..This a very dangerous habit. Just my opinion..