I always brush my teeth right when I wake up (before having tea, breakfast, etc.) as my mouth feels grubby.
DH is a strictly after breakfast brusher. Even when we're at a hotel/resort. He'll go back to the room to brush after coming down for breakfast.
Came up as Which? magazine has an article this issue and seems to be indifferent. So really this is just a curiosity question.
YABU - Deffo brush after breakfast!
YANBU - Pre-breakfast brush is a must!