The reason I ask is that when I pull my 11" x 17" sheets of Photo Rag out of its box, I find that they're NOT flat. For example, if I place a sheet on a flat surface, not all 4 edges of the sheet come into simultaneous contact with the surface underneath -- hence, the sheet isn't flat.
You can try bending the sheet "backwards" i.e., away from the printing surface, to encourage the trailing edge of the sheet not to come up and get in the way of the print head.
Usu. setting the Platen Gap to Wide (via the software driver) is enough to prevent a head strike. But it sounds like you've already done this and continue to get head strikes.