I ordered a ‘Gadsden Flag’ off of Ebay (the flag with the snake and the motto ‘Don’t Tread On Me,’) and it came today folded into eighths heightwise. Therefore its overall appearance looks like a series of sharply creased squares, rather than being flat and smooth.
I would be careful ironing not because of the fabric the flag is made if, but because of the printed image. I would try to keep the iron off of that area, and maybe just try to lightly steam it. At least use a pressing cloth of some type (like, a small towel between your iron and your flag.