Good topic!
I do cull the runts and the ones that don't look good from birth BUT if the deformity was due to an injury I don 't mind breeding from such individual. Goldfish is a good example and the same goes for someof the crappy cichlids out there. Mainly Malawi cichlids.
Selective breeding is something that I like but if that leads to a bad outcome I would rather not go down that path. I love some of the selective bred Malawi peacocks eg. Ruby red peacocks, albino eureka reds, fire fish, dragon bloods etc.... They are some great looking fish. If it wasn't for selective breeding we won't have them.