While a lot is written about how important Magnesium is, I really haven't seen much in relationship to stim meds.
But, as long as you don't over do it, it certainly wouldn't hurt to supplement. Its also pretty easy to get in your diet. Throw a handful of walnuts on your cereal for example.
Yes, in my experience magnesium does help with the negative effects of amphetamines. I take Adderall XR 30mg daily, and I take Magnesium Chelated Aspartate, or Citrate...at night. There are many threads on weather or not Magnesium can potentiate and, alleviate the effects of amphetamines. I've also read that it may also inhibit tolerance.