I am a firm believer of "if you don't see it, don't animate it." No one wants to waste time perfecting something that will never be seen. But! Consider a shot of a character where you don't see the legs, it would be important to still block in the legs, if they're walking or shifting weight. You'll never put the same amount of detail into those legs, but at least roughing them in helps the overall animation. I believe there are benefits to treating the tongue in the same fashion.
When an animator moves the tongue it is usually for one of two reasons. Either it's based on reference, or it's because the tongue needs to move when the mouth is open; movement for movement's sake. This is not a good position to be in (I've been guilty of this myself!), but like any animation, having a good foundation will go a long way toward believable movement. Knowing where the tongue has been and is going to be when not visible gives you a better idea as to where it should be and what it should be doing when the mouth is open. I think it's important to note, when I animate the tongue, it is a rough block, except for when it's visible.
When leading on Snoke I sent this video out to everyone on the team. It is a really good reference for how the tongue is actually moving and the range of motion it has.
The tongue also subtly affects the outward performance. The muscles of the tongue anchor to the jaw and so you do get movement under the jaw as the tongue moves around. This really only applies to the most realistic of characters, but it is a nice detail to add in.
In light of all this, I recommend a few things. First is to attach a camera to the side of your character's head, with the clipping plane adjusted so you have cut the head in half. Now you can see the tongue properly, much like in the X-ray video above. The second is to build up a pose library for your character's tongue. I pose the tongue with the full phoneme when I set up a pose library. Most pose library plug-ins should allow you to affect only the selected objects, so you can do a tongue pass after the fact, if you so choose. Ideally, if you're working professionally, your lead or supervisor would set this up.
Below I have illustrated the tongue position for various phonemes, separated out into vowels and consonants.
Vowels:
AA as in "apple"
AY as in "rain"
AH as in "hard"
EE as in "eel"
EH as in "net"
IE as in "slime"
IH as in "bit"
OH as in "over"
OO as in "spoon"
UH as in "cut"
Consonants:
CH/SH as in "child"
D/T as in "toy"
F/V as in "floor"
H as in "hot"
K/G as in "guild"
L as in "lord"
M as in "meld"
N as in "never"
P/B as in "PB and J"
R as in "roast"
Ss as in "snake"
TH as in "thing"
/TH as in "mother"
Those are the main tongue positions in relation to their phonemes. There's two things to note. First, there is a difference between the two TH phonemes and I think it's an important distinction. The first TH phoneme has the tongue between the teeth and it is a very deliberate TH, the second one is less open and has the tongue pressed against the back of the teeth which is more of an incidental, passing TH. The second thing to note is the N phoneme. I have considered adding in an NG, as they look relatively the same on the outside, but the tongue is doing very different things. For the N, the tongue is pressed similary to the D or T, with NG the tip comes down and the back comes up almost like a K sound. The reason I haven't included it so far is that you have a combination of sounds, the N, and then the K/G, and you could almost blend the two, and have the tongue move through the two positions. I may revise this at a later date after trying things out in some actual shots.
A general rule of thumb for movement in all of these is that vowels are fairly fluid, and consonants are generally a sharper motion away from the pose. Just look at the X-ray footage above and you'll see that all in all, much like the lips/jaws hit or skip phonemes on the way from one to the other, so too does the tongue.
At the very least now you can know why the tongue is moving rather than just moving it for movement's sake. Keep on keyframing!
