Item Balance STDUC vs. Itemasa UCDEF and Itemasb STDUC

To the list...
My customer is trying to use the IM Month End Close option of updating the Unit 
Cost Default with the Stand Unit Cost.  It does not seem to be working for all 
items.

According to the MAPICS FAQ, the CU transaction will only be written if the 
STDUC field in the Item Balance record is zero.  When I zero the field out (in 
Test), change the Itemasa UCDEF to 999.99, run a Cost Rollup, IM Month End 
close, I finally do get the CU.  After the close, I get a UCDEF of 15.999 which 
seems to be right.

What impact on COM and SA (yes, I said SA), will zeroing out the Itembl STDUC 
have regarding capturing costs for shipments and sales?  I think I know the 
answer but wanted to check to make sure.

TIA...
Eric