I try to never use "the finger" when another driver is being an ass. In that case I always just give them a thumbs down and a sad face.
I save the middle finger for people who are being "nice", especially when it is making things dangerous. I find it is the quickest way get them to just go.
I try to be coniderate while driving. Being predictable is safe. Deviating from the rules is dangerous. I think being safe more considerate than being "nice".