1. A method of sorting graphics primitives for rendering into lists representing different areas of a render target to be generated in a tile-based graphics processing system in which a render target is divided into plural rendering tiles for rendering purposes and in which system primitive lists indicating primitives to be processed can be prepared for at least two different sets of sub-regions of the render target area, at least one of the two different sets of sub-regions of the render target area comprising sub-regions of the render target area that are bigger than a single rendering tile, the method comprising: for a primitive to be rendered: determining sub-regions of at least two of the at least two different sets of sub-regions of the render target area that the primitive could need to be listed for in order to render the primitive; using a cost function to assess the cost of listing the primitive in the primitive lists for the render target area sub-regions that it has been determined that the primitive could need to be listed for; and selecting which of the render target area sub-regions that it has been determined that the primitive could need to be listed for in order to render the primitive to list the primitive for on the basis of the cost function assessment, and including the primitive in the primitive lists of those selected sub-regions.