Well, thats not correct, it is claerly mention on the UKBA website that the conversion rate for maintenance of funds should be the rate at
www.oanda.com and the date on your bank statement.see the quoted lines.
cash savings in a account overseas (we convert the amount to pounds sterling using the rate which appears on the OANDA website, which you can finds on the right of this page, and the date on your bank statement), that have been in your account for at least three months before your application.
Maintenance funds - there are no clear guidelines on it in the guidance notes, however the date of application or a day prior to that is advisable.