No, not always. Having blooms on the head is the most common occurrence, but it is not mandatory. Plotties can have blooms in other parts of their body in addition to the ones on their head and sometimes they might not even have blooms on it at all.
On rare occassions, a Plottie's bloom may not develop properly and it will remain closed. In most cases this does not affect their survival or health but it might cause them to be bullied by other Plotties.