A week ago, I bought three 3' lengths of 8mm smooth rod for my Prusa Mendel because I wanted to use LM8UU bearings. After receiving the rod and cutting it, I've fond that they make a huge difference in reducing play between the linear bearings and the rods. With 5/16" rod, you could feel the bearings wiggle. Now there is no wiggle at all! If you're building a "United States customary units" Prusa Mendel and are ordering online and might use LM8UU bearings now or in the future, get 8mm rods. They're only a $1 more each on McMaster-Carr, so it's not a huge expense.

tl;dr: Get 8mm smooth rods. They're the best.