I can't say that I have noticed my hand cramping up when I have been using my mouse. I had it happen a few times a while ago but I have found that with me also typing a lot for work as well, I don't notice it cramping as much as it used to.
When it used to cramp up though I would squeeze my fist and open it again a few times and wiggle my fingers as well and that seemed to help.
I never had any issues with cramping using my mouse, then again I am terrible at just doing random things (like moving the charger on my desk for the 50th time today) while working. I used to keep a grip training in my desk and would do exercises with that from time to time, but honestly, I never kept up with that idea.