For each of your clients, you can define a default currency in the Currency field when creating a new client or when editing the Overview panel of an existing client.
When saving a price your client (under the prices tab), the default currency is the default currency of your client. But if your client works with multiple currencies, you can define prices in different currencies.
When you need to use another currency than the default one you defined for your client, you can simply define it in the currency field of the job (directly in the job creation popup, or by editing the Overview panel of existing jobs).
The system will use the rates you defined for your client for the currency.