The portion of labor that is not applied to WIP is either non-job related production labor, such as machine maintenance, plant cleanup or similar non-chargeable jobs (in which case they should probably be defined as weekly or monthly jobs/wo's which are allocated to COGS), or it's just overhead - waiting around, etc.. This also should be allocated, similar to general overhead that some will allocate back to production. Depends on how general overhead is treated. My opinion is all production labor should eventually be reflected in COGS.
(@JimWoodhead - I guess that was my long-winded agreement with you.)