I guess it would depend on the minerals,etc. in your local water. I understand that usually RO is recommended. I have always used PUR stage 3 filtered water from the tap with reptisafe and have had no issues with the system or the cham.
I think this is where I am very lucky. Our tap water is very good and low in chemicals. Also it is not hard at all. I have tested it out to make sure and it passed with flying colors. I just dump tap water into my 5 gallon bucket.