A complete reinstall from time to time is always a good idea. Be sure to back up you game before doing so. You can find back up and reinstall information on the Technical Help board.
However, it's not necessary to reinstall you game simply because you've bought a new expansion pack. You can install them in any order, really. Back with the Sims 2, it was important to install them in order, but that's not the case with Sims 3. Once you have all the Expansion and Stuff Packs, you may want to reinstall them in order just to keep yourself organized, but it's not necessary for the game to function properly.