Here is an interesting little video from Life Noggin on what causes cramps. 

I personally believe in stretching.  A good dynamic stretch routine prepares you for exercise, and a nice static stretching routine after exercise is a relaxing way to cool down.  According to this article from IFLS, the results are mixed.  It states stretching provides little to no effect for the recreational athlete, and has some benefit for high-level athletes who's sport requires a level of flexibility.  The results are not conclusive, but I say, if it feels good, do it!