‘Real’ currency of liquidity portfolios

As financial institutions usually trade in many different currencies, they also have
to manage their liquidity risk in many different currencies. Nevertheless, they usually
do not hold liquidity portfolios in all these currencies for reasons of cost. Therefore,
the question arises, which currencies and locations are optimal for liquidity portfolios?
In practice the liquidity portfolios are located in the regional head offices and
are denominated in the main currencies (USD, EUR, JPY).