This is something I differ on with a lot of people. Especially with newer auto phenos, I tend to cut their light back near the end of the grow. I normally run 20/4 or 18/6. Around halfway through flower I will cut it back to 16/8.
My reason is pretty simplistic. While a small percentage of the genetics are ruderalis which imparted the autoflowering gene, the rest of the plant's attributes come from the photoperiod side of the family. So it's reasonable to think that changing the photoperiod (and other environmental triggers like temperature) would still have some impact on the plant's natural tendencies toward flowering.
The argument I hear against it is "maximizing time photosynthesizing" but I've never seen any actual data to support a number of grams of lost weight due to the light dsifference. I have seen the additional cost drop off my power bill and can tell you, that number definitely makes a good case for cutting back the light schedule when practical.
That's not to say you can't go a whole grow with 18/6 and get awesome results.
