I've tried it, and don't see it doing anything. I have had a bottle hanging around for a couple of years. I don't trust it not to discolor the iron so I use a plate on the bottom of the iron. I've just found a silicon iron cover and will see how that does. You can take it off and on faster than a metal cover.
I only used starch to make entredeux for heirloom sewing before decent stabilizers for embroidery came out. Everything was starched when I was a kid, and it ITCHED.