I've found that if I put it directly on top, it has a tendency to crack and spit. Putting it on the sides and pushing it in makes for less red-hot shrapnel, even if there's no need for it to coke.
Thank you, @Laertius, @Will W., @51 Papy, and @Gerald Boggs; much appreciated. Good info, and I hope it will help others thinking about the same issue too.