It's a good time to treat.  A rule of thumb--a couple weeks either side of the winter solstice.  The queen has stopped producing winter bees and hasn't started laying for spring.  There's a brood break about this time.  Good time for dribble since oxalic acid cannot penetrate the caps on the cells.  Mites have nowhere to hide.

In the Monthly meeting it was mentioned that the dribble method should be done when you are assured that the cluster will be able to dry off before clustering again for the night.