In all of the books i have read about lucid dreaming (including a few by LaBerge), practicing things such as you mentioned is made out to be one of the greatest benefits of Lucid Dreaming. While i have never experienced it myself, i definitely think it makes sense. Its all about the idea of imagery/visualization. Have you ever had a personal trainer or a coach or something tell you when helping you to try and visualize the thing that you are trying to accomplish? Visualization is widely accepted as a very effective way to practice/learn things. Doing things in lucid dreams is the same concept as visualization, except it is stronger because you are actually vividly creating and experiencing the thing that you are visualizing. Maybe next time you are lucid and practicing rolling, stop for a second before you try the roll and do a little visualization of yourself smoothly doing the roll?