I think an Imperium-wide currency is one of the biggest handwaves in all of Traveller. On the other hand, it's fascinating to try and make it work somehow.
The problem with physical currency that isn't made of something inherently valuable is that it can be counterfeited. While this could be reduced by making currency only on the highest TL worlds in the Imperium (TL15-16), the Imperium doesn't operate like that - it makes it on TL12, which is a TL that a lot of worlds have, both inside the Imperium and outside of it. By the definitions of technology, any equal or high TL world could duplicate the currency exactly if they wanted to.
While small polities probably couldn't produce enough to destabilize Imperial markets in a wide area, there's several places that have a long history of antagonism towards the Imperium and the Imperium wouldn't be in a position to do anything about it - Vargr, Zhodani, K'kree, and Hivers come to mind. The Solomani could do it, but would probably be shooting themselves in the foot counterfeiting as their markets are so integrated with the Imperium's and the Aslan don't seem to be the types to go for that kind of behavior. Indeed if you look at mind-based bent of the Zhodani, you realize that if it weren't for the "romantic" image of galactic war, with worlds in flames and huge starships slugging it out in space, the Zhodani aim of curbing the Imperium could probably be done through judicious use of counterfeiting and psionic influence to create a general sense at the Spinward Marches isn't worth expanding into due to a lack of economic oppurtunities - it'd certainly save lives and the Zhodani could even have a decent relationship with the Imperium.
So perhaps precious substance currencies might work better. Unlike Star Trek there exists no widespread transmutation technology which would render precious substances meaningless. On the other hand, this kind of coinage is only as valuable material it's made from and it's usually constantly being destroyed as the metal is being used for the purposes its valued for. In addition, in an Imperium of 11,000 worlds, "precious" metals might more common on some worlds. Also, there's also the "Spanish Sickness" - if a world where some precious metal is abundant is discovered, the sudden influx of the precious metal would have calamitous effects on markets. However, mankind tends to have this irrational attachment to gold, so that might work - even in a place where everyone is starving people have been proven to still be willing to kill each other over gold. Another candidate is lanthanum. If you go this route, then counterfeiting of another sort exists - debasing. That is, alloying the precious metal with something less valuable to make your precious metal go further. So, at spaceports, you might find "currency checking" that has to occur - everyone bringing specie in to spend has to have each and every coin inspected for purity before they could spend it. The opportunities for graft and manipulation of this interface would probably be good fodder for a few games, in addition.