My 1st selection for a strongback was straight when I left the timber yard but was a lovely curved shape before I had the forms cut out. OK, so don't buy crap timber.
A parent of a boy that I coach runs a reclaimed timber business and offered to help. I was pleased to find a long enough (with some imagination)piece of oregon.
I wasn't allowed to pay for it so the price was right.
As far as I can tell it was very stable and didn't warp at all.