I have always used distilled water to mix concentrated washer fluid as I believed the purity of the water avoided hard water deposits that blocked the jets.
However the manual states, "Do not add windshield washer concentrate and antifreeze undiluted and do not fill with pure water; this could damage the wiper system."
Apparently distilled water has higher levels of acidity than tap water. Could this be the reason for this warning?
The parts guy at my local BMW dealer (the Mini part is an offshoot of it) says they have had many BMWs with washer problems that were solved by using BMW's own washer fluid. Seems implausible but he's right about most things.