Purpose
You may unmark the automatic Revaluation of Currency Accounts that is default set in Company Setup. Then you decide yourself when you want to run these revaluations. At least you should do that on Fiscal Year End.
This is operation recalculates value of your foreign currency savings in GL using current daily exchange rate.
Procedure
For specific Currency: Banking and General Ledger -> Currencies -> Automatic exchange rate update and for default value: Setup -> Company Setup -> Automatic Revaluation Currency Accounts.
Tips and Tricks
The specific currency's setting is stored in the auto_update field of the currencies table.