I think many plants do produce quality seed, but two things to look for are cross-pollination and inbreeding. Unless a flower is self-fertilised, then it needs to be hand-pollinated and protected from 'foreign' pollen sources to ensure that the seed is true. Information I have on sweet corn suggests that if you have less than 200 plants, the strain is liable to deteriorate due to inbreeding. I don't know how true this is; I intend to keep my own seed for one season at least, and possible buy again the following year. the other thing to watch out for, of course, is F1 varieties.