You're going to get a lot of different opinions here. Me? I never leave sharp corners on an anvil. Sharp corners cause no end of grief. Forge a sharp inside corner, do some more forging on the piece and nine times out of ten you go back to that corner and there's a crack. When you're flying around the anvil, doing this and doing that, you'll hang the piece over the edge to straighten it a little and sure as heck, you'll chew it up on that sharp corner. Happens to me when I'm working on someone else's anvil. The last anvil I dressed, I put a 1/2 inch radius in the far side and used it for drawing. On the very rare occasion that I need a sharp corner, I use a hardy block. On the rest of the anvil I put at least a 1/8" radius with one spot 1/4.