Most cannabis strains will asexually reproduce ( i.e. produce male flowers and self-pollenate) if left to grow long enough. White Widow is really notorious for it because it will start to doing at about 70 days of flower, which is also right about the flowering time people want to give it for amber trichomes.
People actually use to make feminized seeds with a process called "rodelization" where you let a cannabis plant go so long they produce staminate ( male ) flowers and produce pollen. Strains that are really resilient to stress ( light leaks, interrupted cycles, etc. ) are choosen because any kind of stress can signal the plant to being to asexually propagate, so they would choose strains that were very stable and wouldn't produce staminate flowers until 80 or 90, sometimes 100 or more days into flower.
Now days feminized seeds are made with colloidal silver or other hormones that shock the plant into producing the male flowers. These strains could still have a tendency to reproduce asexually given certain stress triggers, but because the genes can be selected you can select strains that are resistant to shock as you would with rodelization, but also choose a strain that doesn't reproduce asexually in the late-early flowering stage like White Widow does.
If a plant went hermi for "no reason" that's when it's risky, because you can't predict what stresses caused it to asexually reproduce, and therefore can't prevent them carefully enough. However, if you don't push them past super late flower and keep an eye out for staminate flowers, then growing seeds from hermi plants isn't a big deal. Especially if you're just growing in a small operation.
It gets a little more risky when you're talking about big plant setups. If you can't spend the extra time and attention to examine every little node and flower site for a male flower, then just one can end up pollenating a whole room and ruining a crop. I feel like that's more of a concern for commercial growers who can't sell seedy weed, but it would still be annoying to get your sinsemilla seeded. However with small plants, and low numbers of plants, paying close enough attention and giving them enough TLC makes it unlikely you'd get a massive pollination.
Pretty much all the strains I've grown so far have been hermie bagseed. Never got a hermie so far, but I run very small numbers and keep the environment stress free. One strain I'm working with, Blackberry Kush, is apparently really prone to hermie because everyone I know growing it in this valley has gotten a seed out of a bag that came from one of our dispensaries. Still it hasn't happened to me... Yet.
