There's an experiment you can probably find on YouTube where they leave teeth in water, breast milk, water with added sugar, and breastmilk with added sugar, and showed that breast milk is only as bad for teeth as water, but it is the other food that causes decay. Formula counts as food for teeth.
If you can brush them, great. But don't worry if it is tricky. Brush away the old food before the last feed.
Never ever worried about cleaning teeth after BFing and DD2 fed until she was very very big.
She had one milk tooth filled, but that has nothing to do with BFing and everything to do with her fibbing. It got to the point where her big sister automatically returned her to the bathroom as well as me, because she reckoned 10 seconds was too long to waste on teeth cleaning.
My DS is 7m and the proud owner of two bottom teeth. Everywhere I read says I now need to brush them after his last feed, and make sure milk isn't sitting in his mouth over night. DS is exclusively bf, and I give him a dreamy feed at 10pm so that he sleeps through. How am I suppose to brush his teeth without waking him?! Or do I have to stop the dreamy feed? Very confused!!