i think baffles is right ... windex is made from an organic oil solvent, a weak form of alcohol, and a weak acid ... used in other applications as paint thinner, insecticide, degreaser, and aseptic :P .... i would keep it off your touchscreen
reapeatedly, sure. But we're talking a one time thing here. You can use water was well, just be careful and don't get it inside, just a few drops of distilled or filtered water underneath the screen protector to help it slide.
imho ... if it causes damage with repeated applications then it causes damage with one application ... just not visible damage ... i would stick with the filtered water