Vlad (just my ten pence, Im not having a go here)

No No,

To assume blend shapes to be symetrical in this way is slightly ignorant in the fact that unless you are snapping each point or giving values from one vertex to the next. IMHO would be an absolute waste of time spent.

Eyballs:You dont necessarily have to create an in between for the blink for the perfect arc as eyeballs arent perfectly round and if you know what you're doing this doesnt come into it.

But this is not to say dont use in betweens as they are most useful for 'Sticky' lip edges in particular.

A mix of the two is great, joints and blends, it will take time and effort on your part to do it so be ready for some long hours in the chair!!

Bear in mind the geometry that you'll be using as well, is there enough detail or are you going to animate a displacement map thru an SDK at any given expression. Lots to think about before plunging in.

At the end of the day you need to decide what is best for the end result.

Cheers
Jay