multiple currency accounting and opening balance

[Mark Smith]

Am I correct in the assumption that in order to use Trading Accounts, if
I have an opening balance for a foreign currency (= currency other than
my "reference" currency), that I will have to use my opening balance
account containing my "reference" currency instead of creating a
corresponding reference foreign currency account?

Example:

Most of my accounts use EUR, which is my "reference" currency. Now I am
temporarily travelling, so I found some JPY in my jacket that I want to
track. I have two options:

a) create an opening balance account in JPY, and counter-balance with a
JPY-asset/cash account.
b) debit the JPY cash account and counter-balance with my already
existing EUR opening balance account, using today's exchange rate, thus
automatically using the trading accounts.

I am asking if b) is the way to go as I have tried a) and for some
reason when I do my balance sheet the numbers are *way* off if I set the
price source to "avg. cost" so I am assuming it's because I got some JPY
"free" if I choose route a).

Thanks for your insight!