yes up to the first club/water leaves, thats about as deep as youll want to put them, i have my lights at 18/6, i dont know about your plants as youve yet to mention what strain they are =). i'd say keep it at 18/6 seeing as how thats what theyre used to now, no need to add excess stress.
im not an expert, im just telling you whats worked for me so far. My dealer minored in Botany, told me "K.I.S.S. = Keep It Simple Stupid, the more things you add to the equation, the larger potential for error there becomes." I've applied that, as i like to call it, "Mantra" to my grow and i think im doin ok. just what i think =)