i trim my autos when needed, and so far it has never really harmed them significantly.. sometimes it's needed to get light to the lower branches, sometimes i do it to keep humidity down and airflow up, and sometimes in early bloom, so the energy is focused more on the bigger buds, and not on popcorn..
in regards to when autos flower, that depends on the strain.. if the genetics aren't that great or if they're in a younger generation it can still happen that the non-autoflower trait got through.. it hasn't happened to me personally, but there have been reports of people having to put them to 12/12 as they just didn't start flowering.. after all autoflowering is a genetic "mutation" for sativas and indicas and it takes a couple of generations for it to manifest regularly..