Must I use distilled water?
Yes, it is ALWAYS best to use distilled water in any iron, no matter what a specific manufacturer may, or may not, say. The reason is simple: There's nothing in distilled water to contaminate your iron, and as a result, you will not have stained clothes.
If your iron is already dirty from using tap water... clean with mixture of vinegar and water by pouring mixture into iron, bring to full temp for a few mins, flush through steam vents, and rinse with distilled water. You may need to invert your iron to purge the water from tank as well. Repeat as many times as needed until perfectly clean.
Once clean, NEVER use any other type of water in you iron. Period. Always use distilled water. Hope this helps...