The sharper the better. That is why I use scalpels. My doctor gives them to me. (I prefer the #11) I read an article in one of the quilting magazines where they compared all of the types out there and the one that they liked the best looks just like a scalpel.... hmmmm. When you use those curved ones with the ball they tend to stretch your fabric, get caught if they are not sharp enough and force you to "force" it. Which of course causes you to cut the fabric rather than the thread.
Before you say "no way" - try it for your self.