I use XXX as a place holder for mBTC, the reason why I don't use BTC, but mBTC, is because gnucash can't handle more than 4-digits after decimal point: it automatically change the number, hinders a user to balance in a split transaction. It won't work even if you configure precision to more than 4-digits after decimal.

I know I just replied a post 120 days old, but this is how long it took me to determine to book-keeping my BTC transactions.