Hi Saif,
While posting if you use the cost object as cost center, then FAGLL03/FBL3N report will be in sync with S_ALR_87013611 report.
In controlling if the cost object is different like internal order, WBS element, profitability segment, then you can see it in corresponding cost object reports but as you said FI and CO should always sync.
But for any expense account (P&L Account) the cost object is mandatory to post.
Regards,
Ravi