I have a closed flock now, but each time I brought home new birds I would judge their condition (and living conditions) before making the decision to quarantine or not.  Look them over... do they have lice, or evidence of lice by the nits at the base of the tail feathers?  Are their legs smooth (no scaly leg mites) and not discolored?  Are their feathers glossy and healthy looking?  Are they bright-eyed and bushy tailed?  Do they complain about being put in a kennel to be brought home, or do they just sit there?  When you get them home, check the poopies in the crate for signs of parasites or blood... they ALWAYS poop in the crate on the way home.  If there are any signs of illness, or there's something just sort of "off" with them, quarantine for 2-4 weeks and treat them, feed them, love them. 
One set of girls I brought home had freeranged in horrible conditions - evidence of lice, scruffy feathers... I dusted them and quarantined for 2 weeks until I felt they looked good and could give them a clean bill of health.  
This last set of 3 girls came from a clean home, VERY well cared for, they were molting, but otherwise the picture of health... I said a Hail Mary and put them in with the other girls that night with no ill effects.  
Just take it on a case by case basis, and use your gut instinct.