I worked in a pediatric hospital - we saw plagiocephaly alot in the NICU babies.
Most of the times helmets are only required if the child has serious flattening and it is causing their facial features to look strange.
Typically babies who are hospitalized for long periods of time are most at risk due to the fact that they are in bed all the time. Many premature babies end up having toaster shaped skulls which can seriously impact their looks. Research is still being done to determine if there are any cognitive or developmental effects, but so far it does seem to be mostly aesthetic.
Plagiocephaly develops in term, healthy infants due to being on their back too much. It has become more prevalent after the back to sleep campaigns - it was less prevalent when parents frequently put babies down on their sides and tummies to sleep, because the baby wasn't always laying on the same spot. The best way to help prevent worsening, and to encourage a more rounded growth without intervention, is to really get the baby off the back as much as possible (when awake - babies should always be put to sleep on their backs!). This means propping the baby up, sitting them in a bumbo, carrying them, babywearing with an ergo or a carrier where there isn't pressure on the head, doing tummy time 5 times a day, etc. Basically, if baby is awake, try not to have them lying on the flat spot too often.