Does Shellder learn any level-up moves that you can't live without? Check serebii.net or a similar site for a move list, and you'll see that Cloyster and Shellder learn different moves. Once it's learned everything you want it to learn, that's the best time to evolve it. Your Cloyster will have the same stats (EVs aside) at a given level no matter when it's evolved.
As an aside, Cloyster can learn many of its better moves via TM/HM, so if you just can't wait to evolve it, you're not really screwing yourself over or anything.