After doing so many trial and error it seems that multi currency in OpenERP is a wiered things to tame. I got this fact :
By default OpenERP use EUR as internal default currency, no mater how you change your company default currency, internally OpenERP still reference EUR as the default currency.
Doing a "Settings --> Configuration --> Invoicing" and change the "default company currency" to other currency doesn't mean that you change the EUR reference OpenERP use internally. It just change the currency symbol on your form, report, and the document you print.
If you don't plan to use multi currency feature this is fine. But if you plan to use multi currency feature after you change your default company currency from EUR to others currency, let say for IDR, and then you add another currency (USD as an example) by adding some account and bank account on those currency (USD) you will get wiered number on your journal and account. OpenERP will convert the number on other currency (USD) according to the rate of those other currency to EUR not to your default company currency (IDR).
So if you plan to use multi currency feature and your default company currency not EUR you must change the rate of your default company currency to "1" first before doing any transaction, don't do it in the middle of your fiscal year or you will screw up your accounting numbers.