I just bought a new Ninja blade for my Snapper mower. When I picked it up I looked it over and the blade is really dull. I asked the guy at the store if I needed to sharpen it,he said that contrary to belief you don't need a sharp blade and just put it on the mower how it is. I just looked at him funny and said OK. Now from what I have read and have always believed is that you want a sharp blade. Which is it?