Ansett, while you are right that national governments budgets don't have the same hard constraints as individual budgets, this does not mean that the Imperium can just spend what it wants and everything will be fine. If Imperial spending increases too quickly relative to economic growth there will be inflation. The main way to tamp it down is to make sure spending does not exceed taxes by too large a margin. If the inflation is not high, this is usually a good thing, but if it gets too high it could be really bad. Since the Imperium is huge, there could be different inflation in different areas: even if spending overall is under control, if they Imperium spends too much in a certain region, while not taxing there enough, there could be local inflation. On the other hand, if there are too many taxes and not enough Imperial spending in a region, deflation is possible, which tend to lead to recessions and depressions, and underutilization of economic capacity (because people and machines are unemployed). (These local inflations and deflations could be reduced by local currencies, but this looses the advantages of a single currency, which I won't go into here)
The Imperium is set up like a weak federation, or the EU, with a single currency, but many sub-units who (apparently) make their own fiscal policy, but have a unified monetary policy (the Credit). There are feudal lords, but they don't seem to control that much of the total production (of course, as individuals they seem to be pretty rich). So planetary populations have most of the wealth and planetary governments seem to have their own tax systems. The potential revenue streams for the Imperium are: Imperial possessions, taxes on planets, taxes on nobles, corporate taxes, taxes on interstellar trade (requiring an infrastructure which we don't usually see in Traveller play), and of course seigniorage, which is what Ansett refers to. There are lots of other potential minor revenue streams, like selling mining rights, or fines for legal violations. Some interstellar polities might make their money by conquest and plunder instead, but this requires continual expansion, and comes with its own problems. It doesn't seem to be significant for the Imperium.
Getting all this sorted out would require a lot of work, so I think we all just have to assume that something of this sort if going on behind the scenes, and only go into the details when it affects our adventurers.