The statutes speak to the administration of assets. The full statutes for PA are here:
http://www.legis.state.pa.us/WU01/LI/LI/CT/pdf/20/20.pdf
While the estate tax cannot be levied until there is a decedent, I am not seeing anything that deductions from the tax could not occur prior to a declaration of death.
Again there is no question of anyone that "waits 6 years to bill (or to be paid) for services rendered." The cost is incurred, a bill sent, that bill paid, and when, in probate, the payment is credited against the gross amount of the estate for tax purposes. (That is basically how deductions work.)