I think he's saying to mag them for cracks/micro cracks. When we weld on our derricks, we X-ray the welds the first time, then have to mag them to get re-certifications. On non-load bearing stuff, we are required to mag new welds.
I think having your welds magged would be overkill, but that's JMHO. It sure wouldn't hurt, of course, to be positive they're perfect, as that is a criticle part!
Looks great!