Having a range of windup keys is a must if you have a clock with an undocumented key size, or if you're considering clock repair as a hobby.  The windup shafts are difficult to measure, and the difference between sizes is small.  It would be easy to use the wrong size key if you don't have the full range of sizes at your finger tips.  The wrong key size could easily slip and damage the edges of the shaft.  I recommend getting both the odd and even sizes, so that you're sure to have the precise size.  I can't say how well these keys will hold up under moderate to heavy use, but so far so good.