Many people believe that a sharp blade is more dangerous than a dull one. This is absolutely untrue. A sharp blade is much more likely to cut where you want it to. A dull blade requires more force and is more likely to jump out of the cut (slip). It is when a blade slips that you are most likely to receive a deep razor cut. blades frequently because a sharp blade is less dangerous and simply works better.
There is nothing sharper than double-edge razor blades. But holding a double-edge blade free-hand is dangerous so I don't recommend that you use them except in tools specifically designed to hold them such as razor planes. Single edge razor blades are the next sharpest blade to a double-edge blade. They are the primary tool to use for cutting and trimming covering and thin sheeting (3/32" or less thick). Use a new blade when trimming covering because the last thing you want is a dull blade to get caught on the covering and tear it. Single edge blades are also great for picking up small thin parts such as washers that get dropped on the floor.