I don't think its unreasonable to specify distilled or deionized water for ethanol-based disinfecting solutions. Tap water is an undefined product, it may contain random dissolved salts, depending on the time of the year, or the location. Above, someone has stated that it might be more microbiologically clean that the DI system, but even if that is true sometimes, its not under the facilities control, so if its important, you never know if its always the case, for all times at all facilities. You're not going to be allowed to use tap water, if there is an auditing process, it just looks sloppy. And being slightly sloppy, when it doesn't really matter, doesn't make you look ingenious, it makes people wonder, "What other corners do they cut, 'round here." c.f. : http://www.chemicalforums.com/index.php?topic=29560.msg112551#msg112551
I can see your reasoning, but still find it weird to use a pure product (or more expensive product) like demiwater to make 70% ethanol solution that you use to clean your working bench for example.
I really find it weird.
I also dont see the impact of the random salts, dont see how that would matter for 90% of the usage of the 70% ethanol.
You said that the tap water is not under the control of the facility, but be honest: how many facilities test their demiwater on a regular base to check if its still up to standards?
I have worked in a lab for a few years now and there was never (!) a real control on the demiwater. And its in many labs the same thing.
Another thing: many labs have a system that uses a plastic tube (that is attached to the tap (valve)) to fill their bottles with demiwater, this plastic tube is often used for many months/years without ever being cleaned. I am not sure you know a lot about biofilmformation, but I can tell you that those plastic tubes are often filled with bacteria and other "crap".
(I have also read literature in which is stated that in some labs they even found algea(!) growing in the systems that were used for demiwater or even worse, miliQ water systems.
If you indeed use a regulary checked demiwater system I can see your point, but for many of the labs this is not the case.
About using distilled water: is that not really really expensive?