How big of a city are you flying out of?
Unless you have a very young child in the group and get family boarding (and even then they sometimes only let 1 adult plus the young child in for family boarding) - i would suggest getting EBCI if it is imperative that you sit together. I have often gotten B boarding pass numbers when checking in at 24 hours and you MIGHT not get 6 seats together in B boarding.
When do you go? You get 'in line' for EBCI the earlier you purchase it. So if you're leaving in just a couple days it might not be worth it since most of the EBCI folks are already in line for their passes.
I always tell my kids that we're going to try to sit together - at least 2 and 2, but there is a possibility that we might sit a bit away from each other. You have to know it's a possibility, especially if you don't do EBCI, and be okay with that potential. If it's going to ruin your vacation - then spring for the EBCI.