For the wood to reach stabilization. It’s very important if you have a thick clear coated bass in a very cold shipping environment to let it sit inside your house in its unopened shipping box over night. That way the bass comes up to room temperature very slowly. Speaking from experience, you can easily crack the clear coat finish if you expose the bass to a sudden temperature change (say sub zero C outside and 22C inside). It’s a real thing, mostly in colder climates.
You try to adjust each one about the same amount, no need to be exact, but you don’t want to crank one down and not the other. My Alembic basses have dual truss rods.
Yes, Greco made beautiful bright white birds, here’s my 1990 Greco white bird along side my 1977 Greco burst…
View attachment 4930729