Tap water can contain minerals/substances that will contaminate a tip, reducing it's lifespan. Distilled water eliminates this issue. And as mentioned, it's cheap and lasts a long time.
Also, use appropriate sponges for soldering irons, as there can be chemicals in a household sponge that will cause corrosion on a tip, such as sulfur (tip covered in iron sulfate won't hold solder worth anything). And toss dirty sponges, as the flux can become a problem (increasing concentration, corroding hot tips).
All this said, the brass wool cleaners are worth a look IMHO, and aren't all that expensive (maybe $10). Less hassle, and the results are so good, I'm not turning back.