I like to use the Belgium philosopy. Breed a good family of birds, obtain stock that beats you in the races, cross them into your family. If the crosses are better than your birds, keep the newbees and rid the rest. I try and only introduce either performance birds or lineage from winners with good conformation. 30 or so birds will leave my loft this year, one will enter to add to my breeders. She is a Vic Miller bird crossed with a daughter of Scott McCallisters 2000 snow bird winner. She is also a half sister to my best breeding birds. I agree that big, sick, and poor performing birds should go. I do not know who said it, but I read something that made sense to me. If you do not like the looks, feel or performance of a bird, the pedigree means nothing. He also said that winners look good to everyone.
Randy