Trading accounts - when do they make sense?

[Mark]

Hello,

I have played a little more with trading accounts and now have some 
fundamental questions.
First the background story:
I live both in the US and in Germany. So I have bank accounts in both 
EUR and USD and also expenses in both currencies.

For handling those accounts, I have two options:

 1. For some expenses (e.g. insurance), for which I want to US vs DE
    keep apart, I set up two sub-accounts Expenses:Insurance:DE and
    Expenses:Insurance:US (and possibly more accounts as sub-accounts to
    one of these two)
 2. For other expenses, which I don't need to keep separate (e.g.
    Groceries or Dining), I just have one Expenses:Groceries and book
    both USD and EUR transactions in there

I am using trading accounts for my GnuCash file, although I am not sure 
if that really makes sense for this scenario. What I noticed was that 
after entering a bunch of groceries transactions, the balance on the 
top-most trading account was negative, although I had only entered 
transactions from a EUR checking account to the USD Expenses:Groceries 
account. No other transactions were booked in any of the trading 
accounts. And all of the transactions in all the trading accounts were 
in one direction only (EUR converted to USD).

So now I am wondering

  * Does it make sense that the trading account balance is not 0,
    considering that I am not really buying and selling a currency so
    far (all transactions were EUR->USD only -so just buying, not
    selling yet)?
  * Does the trading account affect any of the reports (net worth bottom
    line, etc)? Or is the trading account balance not really considered
    anywhere?
  * Does it make sense to use trading accounts in my case?
  * Would I be better off to keep my groceries separated into
    Expenses:Groceries:US and Expenses:Groceries:DE as well in order to
    avoid multi-currency transactions; and use multi currency
    transactions with trading accounts only for those cases, for which
    it really does make sense (although I am not quite sure which
    transactions that would be)?

I tried to google this, but except for Peter Sellinger's tutorial and 
the GnuCash basic trading account tutorials, I have not found much...

Thanks,

Mark