What Thickness Drywall should I use?

I am finishing my bonus room. I plan to use 1/2 on the kneewall. From there it goes up at a 45 degree angle 56 inches. The top ceiling is 42 inches wide. I am going to attach the drywall vertically to avoid to many joints plus the scrap works out much better and I would be using more manageable sizes. The trusses are on 2 foot centers. A friend said I should use 5/8 for the 45 and ceiling. Would I have any problems if I used 1/2 everywhere?