While I don't know anything about ground fills, assuming you are seeing the pixelization in Fritzing, export the pcb to gerber file->export-for production->gerber and view the gerber files in a gerber viewer (I use gerbv from the geda project). That is what the production house will see, and if it is jagged in the geber view then so will be the board. Note gerbv has varying qualities (lower to speed up rendering) so you probably want to use one of the higher (and slower) ones such as high quality.
Peter
