i've grown probably around ten or so fem seeds from as many breeders from people like dna, reserva previda, sagarmatha, cali connection, and a few others.. everyone has been a girl, and i have never had any hermie problems.
i am not sure what people do in order to get hermies from their fem seeds, but thus far i've yet to have any issues at all, knock on wood of course, lol..
i think the main thing is to stick with some of the breeders who seem to have their fem game down, which not all of them do obviously.. i've heard of lots of problems from greenhouse fem'ed seeds as well as say barney's farm, same deal.. i have also heard that th seeds had some issues with their mk ultra fem's, but i've only grown their regular mk ultra and loved it... i also grew a couple of fem's from dr greenthumbs, and have had great experiences with all of his gear for sure.
do your homework, learn who's fem's are known to not have problems same as who's don't have any issues, and you shouldn't have any issues if you keep them happy same as you would with a non fem'ed seed..