NOOOOOO! NEVER
windex is good for plain glass, not coated glass like eyeglasses or iPads and such. It contains ammonia and phosphates. It will not harm the glass but will remove AR or oleophobic coatings over time.
Spit or water and a micro fiber or eyeglass cloth will remove everything and not remove the oleophobic coating.
Isopropanol is less severe, but not recommended either by apple.
Muc off is a non alcoholic cleaner like the non- Purell hand cleaners. I think.
http://www.partinfo.co.uk/files/382A Muc off stocked CDU with LED MSDS.pdf
From the horse's mouth:
http://support.apple.com/kb/HT3226#iphones
iPhones
iPhone 4
Clean iPhone immediately if it comes into contact with any contaminants that may cause stains, such as ink, dyes, makeup, dirt, food, oils, and lotions. To clean iPhone, unplug all cables and turn off iPhone (press and hold the Sleep/Wake button, and then slide the onscreen slider). Use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Dont use window cleaners, household cleaners, aerosol sprays, solvents, alcohol, ammonia, or abrasives to clean iPhone. The front and back glass surfaces have an oleophobic coating. To remove fingerprints, simply wipe these surfaces with a soft, lint-free cloth. The ability of this coating to repel oil will diminish over time with normal usage, and rubbing the screen with an abrasive material will further diminish its effect and may scratch the glass.