I usually try to pollinate (and tag as pollinated with a twist-tie) the lowest branches, in case the main cola's are ready before the seeds. Then, I can just snip the main stem above the 'seed-line' and keep the rest of the plant in flower till seeds are more than ready. I've harvested too early plenty of times, ending up with half-baked seeds. (yellow, immature, too small for viability...) So now I give them some extra 'quality time'.
Good luck with 'em.