I would not use the iron x on the wheels while fitted to the car and if I did I would as normal drive the car a little way using the bakes to keep them clean and reduce the rusting on the discs. It is supposed to be safe as it is neutral PH but they do say this:
"Will Iron X affect brake calipers ?
For most high end calipers it will not affect them however we have heard from customers in the past that Iron X can affect the paint colour of low quality acrylic painted calipers. For this reason we advise you to be more careful not to spray on them."