How does a math degree help in electrical engineering graduate studies?
Yes, it would help if you study Applied Mathematics. 

However, how much it would help depends on your intended field of study. If you plan to go into Computational Electromagnetics or Digital Signal Processing or similiar fields, then it's great. If not, I advise you not to invest more than one semester in studying Math.

To give you a perspective: I graduated with a BS (GPA:3.5) and MS (GPA:3.5) in EE only. I took the subject Math GRE and scored in the 30% percentile.