I agree, using the ImpCr would be 'easier' than planets making their own currencies ( both for game simplicity and 'making' the currency ).
As Tim said, the big questions would be the money supply relative to the size of the economy...too many Cr and the value dives, too few and it will rise. A further problem is that very few planets would have the capacity to mint new Cr, so the supply is fixed ( too high or too low probably ).
Of course, there is an exception to all this, but I doubt there are too many rulers who took the 'extra' Cr that were available and only released them as required. ( cash also 'wears out over time..even coins..notes only last a few years at best ).
It is also worth noting that in a modern economy, only a tiny proportion of the 'money supply' is actually cash ( ~5% is typical ). Most is just electronic transfers and the like.
Further problem .. banks *cannot* cover all the money deposited with them in the near term...after all, they've lent most of it out. The bad part of this is that when things go bad, people tend to make a 'run' on the bank, trying to withdraw all their money. ( this was a major cause of the great depression..the banks ran out of money ). The modern solution to this is that central banks allow individual banks to borrow cash from them, something a planetery bank probably can't do, so as soon as people start to think things are going bad, the whole system will probably collapse.
There is a reason economics is known as 'the dismal science'.