I have had engines rebuilt over the years and never had it done. Just a visual inspection. I kept my fingers crossed and so far I never have had a problem, but there is always the first one.
I have a couple of 340 T/A blocks that I might be selling in the future and I'm thinking to get a top price on them I would have to have them magnafluxed first.
Just remember that the the inspection is only as good as the person's experience is who is doing it.