In our world, the Fibonacci sequence, a series of numbers that approximate a logarithmic spiral, appears constantly and unexpectedly in nature, like in sunflower heads, the sprouts of a pineapple, and in pine cone bracts. Many ancient civilizations -- from the Mayans to the Celts -- depicted spirals in their primitive art. Galaxies, hurricanes, seashells, fingerprints; spirals are coded into our universe.
It's hard to say if spirals will ever mean anything specific or significant in what's left of Game of Thrones, or if they're only meant to draw a connection between our physical world and that of Westeros. But that Benioff chose to emphasize their significance means we'll probably, at the very least, see some more elaborate White Walker body art by the time the show is over – full Fibonacci style.