Of all my sales letters and squeeze pages, I only split test the top 10% that get the most traffic. The whole point of split testing is to have two versions of your sales letter or squeeze page, Version A and Version B.
Version A should only be slightly different to Version B. You might change just the headline or one header graphic. Then by using tools such as Google Website Optimizer, you can track the results and find out which version brings in most opt-ins or sales. Usually small differences can mean a big difference in your overall conversion rate.