Question

Why, when writing down an orbital diagram representing the VALENCE electrons of Bi, do you not include the 5d10 shell?

Answer 1

the valence level of an atom is usually defined as the highest occupied principal level (highest n-value). The condensed configuration of bismuth should look like:
\[[Xe]6s^25d^{10}6p^3\] since 6 is the highest occupied level, only the electrons in the s- and p-orbitals "count" as valence electrons, so we ignore the 5d's.

Answer 2

Then why, when writing the valence orbital diagram for Cu, do you include the full 3d orbital? (the memo to one of my tutorials has that as the answer)

Answer 3

the d-level in copper has just been filled, at the expense of the 4s level, so the 3d is in the proces of being filled in copper, but has already filled in bismuth.