Cost Element Creation against COGM & COGS

Dear Experts,

Some person suggest that while creating COGS GL Codes in FS00 cost element should not be assigned.

According to him if this is done then COGS will flow to COPA and thus COPA profitability report will be wrong.

Can you please tell whether this correct, how ever in our system COGS a/c also maintained with cost element in FS00.

Regards,

Alok