it is up to you? but if certain they are all seeded, and you want smoke then yes time to chop..but that's just my opinion?
I don't know how bad it is...but when I've smoke some with seed in it its not as good as "sensimellia" seedless weed.
another note...have harvested a plant( I moved so had too) 4-5week flowering and i got high off it! shocked me? but i wasn't going to complain, was just gonna make some oil out of it but it smoked fine, so just kept on smoking it!
yes I would def leave one to finish, so you can see what would have happened, to the rest, if you leave em to finish with seed in it