If you do so, you must first clean off the old grease. Rubbing alcohol works fine for this. Use the 91% stuff if your local store carries it.
There are two primary cautions to observe. First, do not apply unnecessary downward force to the CPU. It is crackable. Second, do not overapply the new grease. The stuff can be conductive. If it runs off the chip onto the pins around the edge of the chip it can cause shorts and kill your 601. The grease is just filling the imperfections from flatness in the 601 die and the heat sink. It doesn't take much volume to do that.