If you free range your pigs in an organic manner your pigs will not need wormed as the earth will naturally fail to support the woms that could infect pigs. Also, if you do keep them indoors, you merely need to 'rest' the barns between rotations and ensure they are cleaned really well when empty. If you are going to keep putting pigs on the same area without resting it, they will only need wormed if you are going to keep them for breeding.
Kind regards,
Andrew.