Thats not true for outside grows. For some reason the plants seem to deal with minor light if planted and grown outside. It has to do with the hormone levels in the leaves of the plant, to much of one and it goes into flower, to much of the other and it reverts to veg. I haven't done the research but if I had to hazzard a guess as to why outdoor plants can handle small amounts of light (remember we got stars and a moon outside) it would be because of the sheer intensity of the sun during the day.
Plants that hermie do so because of a genetic disposition to the hermaphrodite trait. Sometimes stress is required to bring the trait out of dormancy (Hermaphrodisim is actually a defensive mechanism developed by the plant to allow it to pollinate itself to help propagate the species) this trait does have the potential to be expressed at anytime though. Plants that fall to hermie easily are looked down on by growers because of their unpredictability. All plants can be forced to hermie through one way or another.
So to answer your question yes a plant can hermie at anytime depending on its genetic disposition to do so. Also if you are growing inside then yes you need to worry about light leaks, if you are growing outside then that is out of your control unless you wish to purchase a tent or grow bags or something but they aren't needed unless you are trying to accomplish two harvest in a season.
