When entering the US in LAX, are CBP still taking customs declarations?
Yes and no - You will be still be asked to declare the value. But you need not to itemize.
I would be entering on a non-immigrant work visa and prefer if I did not have to bring an inventory of shirts and underpants with me...
I believe CBP would be lenient in this situation. Even technically you are not an immigrant, because work visa is generally dual-purpose, CBP would treat you similarly as new immigrants. But just keep in mind - be reasonable, i.e. CBP should not expect the baggage is full of new clothes and stuffs...