I haven't had this done, but at my 09 filly's first vet visit (she was 18 hours old), the new/young vet told me I'd need to get it done. My filly was large and took a couple of days to unfold. She is now a yearling with very nice, naturally straight legs.
I had it done to a foal i bought. her left front was turned out 75 degrees. it was the best thing i ever did. she is not 6, she still turns out but the correction was extream and i am so glad i went with surgery.
The first horse I ever bred had it done twice. But he was a bit of a special case as he was rejected by his dam and did not get all the colostrum/milk he needed in the first few days (10 days at the vet school fixed that problem, but not the missed early growth). Under normal circumstances he likely would have straightened up on his own. His legs are perfectly straight btw. I have no problem with the surgery (it is considered fairly routine here in KY) for a performance horse, but definitely think it should be disclosed in breeding stock.
Years back, we had a young horse that was valgus in the hocks. The exterior growth plate did not grow at the same rate (slower) as the inner portion, and the results were an outward angle when viewed from behind. The outside of both hocks had the stripping done. The result was very successful, and the adult horse looks normal.
I had my filly's knees stripped at 4 weeks. Maybe she would have straightened on her own, but maybe not and I was not willing to take the risk. She is now at three months and perfectly straight - you cannot even see the small line/scar on her knee. But, it is a PITA to administer SMZ and bute to a weanling and re-bandage one that amuses herself by taking them off!