Unless you are obligated to remit an exact percentage rate after each and every payroll, thus treating it like employer share of FICA, at the end of the year it would be under payroll expense anyway.
If you receive a monthly bill or record a memorized withdrawal from checking you would post that as an expense when paid, not when incurred - I see the difference being that WC, although required, is optional as far as rates and you can even legally self-insure in some states.
The way unpaid payroll expenses that are posted with each payroll run work is it creates a current expense as well as a future liability. Consider the FICA or FUTA you paid n January for 2018. Take a look, that already was posted as 2018 expense prior to you paying it now. WC Insurance can work the same way...or in my situation, for 50 years or more we have always posted WC as Insurance and not ever part of payroll
