I don't like curled fins. I know of one breeder that consistently line bred his fish at 3 -4 months of age. His fish were near perfect, but a curly ventral showed up at around 7-8 months of age. That is one of the reason why I don't breed my fish too young. Curly fins or other undesirable traits might not show up until the fish is older. What good is a betta fish that only looks good at 4 months of age when they live for years?
Once they're curled they tend not to straighten again although it does depend on the severity of the curling. One of my HM's fins started to curl before I used IAL and his straightened back out fine. My CTs never did.