For dealing with currencies, remember that you need to always remember not just what currency the amount was entered in, but also what time it was entered, and what the rate of each currency was at that time. Also, accountants are not forgiving when it comes to "inaccuracies" in amounts. If an amount is entered, you have to store it as it was entered, and not convert it first, because afterwards you won't be able to guarantee that you can get back the entered amount just like it was entered.
These may sound like obvious things, but people do sin against them in the real world.
