I'm in the same zone as you are, 8 - Summerville, SC. I usually prune around the middle to end of February because in our zone we have unexpected warm ups and if you prune any earlier than that you risk stimulating the plant into producing new growth when one of these warm ups occur which you don't want because of errant freezes. In my experience, unlike other plants, hydrangea is not as sensitive to soil temperature as other plants are - they rely more on ambient temperature for clues as to when to wake up.
