you can never tell if they are going to hermie later on on you or not.. i'd think that if you were messing with the light hours in flower instead of veg, you'd have much more to worry about ...
i'd say so long as they are looking healthy and what not so far, why kill them?? let them suckers live... and if its been too hot and that is why you've been running the lights like you have been, try running the lights at night time when it's much cooler outside instead of during the heat of the day.. if you want to run them for 18 hours a day, just turn them on at around 7pm or so, and take them 18 hours from there, that way you're avoiding the hottest parts of the days, this should help keep the heat down..