This varies per payor. I hate to be vague, but each payor is different. It's best to talk with the insurance company and see what they accept. For some, we've found that it's best to send the preauth with the claim for the buildup. This way they’ll approve the crown and pay for the build up without any question. Just in case they do not want to pay for the crown without the buildup.
But something to keep in mind... always bill for exactly what you do. My first instinct is to say that the buildup should be billed on the date it's completed, but again, check with the payor.

D2950 should be bill the day it is rendered to avoid denial for inclusive reasons, and you should always bill for what you do on the DOS. D2950 is considered a stand alone code, and is commonly denied for inclusive procedure when coded with a crown code on the same DOS without a seat date. Payors can not pay for the build-up unless a crown is seated because the build-up was rendered for retention of the crown. By notifying the payor of the seat date this can usually reverse this denial as they are looking for the seat date to approve the crown needed retention to be seated. However, there are some payors that can still deny the build-up if they do not think the crown needed support for retention. It is best practice to document 50% of natural tooth structure was destroyed by decay and retention is necessary for the crown to have a good and long prognosis