distilled water doesn't have anything in it but water. If you add anything else to the batteries other than water you run the risk of causing a secondary chemical reaction in the battery that will almost certainly have negative results, ie. sulfating, calcifying or just plain lowering the Ph of the electrolyte.
I agree with Tucson, I used to work at a plant that made deionized water for the U.S. military. They used it in their batteries. Distilled and deionized will both work. They are just two different processes that get the same end result, pure water. The last time I bought some from Wal-mart I paid $0.50 a gal. Batteries start to die the minute they are manufactured. How you take care of them determines how long or short that process will be.