Our son also had mild plagiocephaly and had moderate tortocollis. He significantly favored one side as an infant. It was all we could do to correct this behavior and was a source of frustration along with some other early challenges. His head was noticeably flat in the back on one side. We got three opinions. They ranged from "get a helmet now, what are you doing!" to "it's very borderline; your call." His primary care physician, fwiw, told us we should do something about it too, purely from a cosmetic standpoint.
There was a study done by NIH I believe that suggested the helmets don't do nearly as much as the companies advertise. This was released about a month after our decision. We brought it to our pediatrician's attention. He laughed and basically said "you guys made a good call ignoring me."
A couple other factors: It would have been a considerable out of pocket expense for us, several thousand, as insurance didn't cover. He had many follow up appts with doctors due to monitoring an early challenge that also, incidentally, went away or never was. Having a helmet on his head was like salt in a wound for new parents who already felt stretched to max emotionally over their baby.
In the end, we opted against it. It seemed superficial compared to the other ongoing care requirements. He grew out of the tortocollis. His plagiocephaly improved. It's completely unnoticeable now to everyone except us and the woman who cuts his hair. She actually smooths it out by letting one side grow a tiny bit longer. If he shaves his head, yeah maybe you'd see it if you were looking closely. If he shaves his head, that's on him. If he loses his hair one day, he'd have to lose almost all of it as it's really in the back part if his head.