The bottom line is, if you buy store bought distilled or cheapo De-Ionized water, you don't really know what you are getting unless you check it for purity. Some store bought distilled water can be really dirty, and same goes for cheapo De-Ionized from auto parts places, etc.

Does it really matter? Probably not. But if one is going to get that involved in the purity of their water, and don't want to fork out $75 for a water tester, the only way to be reasonably sure of EXACTLY what you are getting without measuring it yourself is to buy ASTM graded water from a chemical supplier.

ASTM Type II Laboratory grade (less than 1µS conductivity):

http://www.aaa-chemicals.com/wade1ga.html