As long as you buy open pollinated varieties from a reputable vendor and save seeds you should get the same variety form those saved seeds. HOWever squash are promiscuos and will cross readily with others in the same species. So if you grow halloween type pumpkins (C. pepo) they will cross with zucchini, crooknecks straightnecks,and other winter squash in the C.pepo group. I f you grow more than one variety or have a neighbor growing a different variety you will need to hand pollinate and bag the blossoms to get pure seed. Hybrids of course tend to revert to parent plants so avoid those if you wish to save seeds.