On Kent Trammell’s Creature Sculpting for Production course (which is now a part of the CG Cookie citizen membership), he sculpted, then retopologized the creature, leaving a gap for the mouth. Then, after doing the retopology on the rest of the body, he set up some basic pose settings that allowed him to “open” the mouth. With the mouth open, he did what Ognjen said and created a separate mesh for sculpting the inside of the mouth. With that complete, he continued the retopology on the inside of the creatures mouth, this time over the newly created sculpt.
From my understanding, this is just one possible workflow.