Having worked in a biology lab, and mixing many different chemicals into solution, I would suggest using the distilled (aka deionized) water. As said before, you can purchase the water relatively cheap.(posted 8423 days ago)In the research laboratories. It was mandatory all solutions were made with AT LEAST double distilled water (taking distilled water and distilling it again). Distilled water still has traces of minerals or particulate matter... not good when the solutions are to be used in experimental data which are eventually published. The double distilled water, however, was provided by the university and was not to be wasted.
Many research laboratories used the extra precaution of nano-pure water (not to be confused with purified water through a filter). Basically the same as double distilled water, the nanopure water also is free of dissolved gases such as CO2, Nitrogen, etc... that distilling doesn't address (distilling can separate solids from liquids or liquids from liquids, not gases from liquids).
Basically, I use the regular distilled water and it works great. I know you can purchase the nanopure water, but it's very expensive stuff.