A t
You are correct. The ice initially forms on the leading edge and then works it's way back.
The problem with using the wing anti-ice before there is an appreciable build up is that the water freezes further back on the wing. By allowing a build up to occur and then switching it on. the ice that is in contact with the wing melts and then the ice breaks away in chunks.
http://icebox.grc.nasa.gov/ext/galle...icing_wing.jpg