This question actually doesn’t have a simple answer. It’s actually pretty layered.
Let’s look at facial animation through the steps of animation (at least the process that I’m accustomed to). I treat the face as part of the drawing when it comes to blocking. It finishes off the pose and has everything to do with the appeal, the line of action and the intent. The face has to carry what the body is intending. I like to move almost everything that can move (brows, lids – upper AND lower, cheeks, lips) out of their zero pose. This allows the character to be different than normal so it seems a bit more alive.
The second part is dealing with blocking plus. How does the face go from one pose to the next? Is it leading the action or is the character thinking before he does something? This is setup a little in the anticipation but mostly on the breakdowns and in-betweens.
Did you notice one thing that I haven’t mentioned at all yet? It’s lip sync. I personally think that lip sync should be done last. If you can make a character emote and TALK without moving his lips, the sync will simply be icing on the cake.
As a director, I would ask that no lip sync be put on the blocking pass at all. I felt like it would do two things. First, it would distract the heck out of me. I wouldn’t want to look anywhere else because those dang lips were flapping! Secondly, it could make the overall animation weak. Remember what I said about making the character talk without sync? This is where it comes in. If you spend all of your time doing sync before the rest of the animation is done, you will lull yourself into thinking the animation IS done. You may not even notice the character is out of balance and could lose out on an opportunity to make an OK scene great.