Taxation of one sort was ubiquitous in the civilizations of Old Earth. It has changed quite a bit the last 10,000 years, and will likely do it again.
How do taxes get paid between systems in the Civilized Galaxy? Even an omniscient admin AI needs raw materials, and the governments that pop up invariably seek new and untapped sources of revenue. One of these is, of course, other systems.
The particulars of taxation or its equivalent varies considerably For instance, the NoCoZo is simple - no taxes, at least not on a collective level. It is true that member organisations/systems may pay a certain fee to participate in the Board, but this is almost always marginal. How they solve things at home is up to them. One approach within Metasoft is to charge licence fees for running the Metasoft software - if you don't pay, you won't get upgrades and become incompatible. Since parts of it are "meterware" that depend on linking up to more central systems for some operations, it is not entirely impossible to keep track of licence dodgers. The Utopia Sphere is after all an utopian socialism to a large extent - the machines do all the work - so taxes are fairly irrelevant there for most modosophonts, though the AIs running the system still trade internally with each other. The Efficiency Maximization Paradigm is somewhat similar: the Paradigm itself (a group of virtual entities) is the government, taxing the natural world by building stuff. The inhabitants of the uploaded worlds are however free to implement whatever system they like. Other meta-empires, Sephirotic and otherwise, have a variety of approaches, sometimes within the same over-arching meta-empire; likewise the myriad minor independent polities show a variety of strategies.
In order to make it attractive for a system to pay tax to another, there are several approaches. The simplest way is the medieval, which basically constitutes extortion. The second easiest is guaranteed protection. However, for outlying systems a defense fleet arriving twelve years after the nanite swarms had their fill may seem rather pointless. The most complex, and probably most durable, is the providing of public services by the "central government". These might take the form of territory-wide broadcasts, appellate courts at central system, institutions facilitating trade and commerce and so on.
If an organization is relativistic, then taxes make sense mainly on the local planetary and or sometimes system-wide level. A hierarchical tax system would exists in some cases: you pay the life support/infrastructure tax to your orbital and a system tax to the system authority, which in turn pay a tax to the region/sector authority, which pay to the central government. Of course, much of the money may not need to leave the system in any form but can be re-invested there, giving the governing organization a strong economic hold.
An interesting result of this is that worlds that are not much on their own, may earn a central position because of astrographical and political factors. The Algol Broadcasting Foundation for instance, has made Algol Orbital the place to go for media people in numerous surrounding systems.
The tax systems themselves, except for snatch and grab extortion, have adapted to two main circumstances. Firstly, the distances involved would make a single currency pointless, so the tax must be paid in commodities or information. Secondly, even mortal baseline-equivalent bionts tax-dodge, often with great success. Immortal super-sentients and transapients may do the same. It is not uncommon for a century-long tax-case to be thrown out of court because the defendant transcended...
How does one prevent, or at least minimize tax-dodging? One common approach is pulling an Occam. "If two solutions seem equally probable, its often the simplest one that is correct." Intricate taxation systems are usually impractical and easy to scam, so the central government is best served by keeping the tax system as basic as possible:
'Goedel & Clones, interstellar experts in taxation law. A firm of lawyer-beings finding loopholes even in the most basic systems, working across the Wormhole Nexus and using massive AI support to construct their cases. "No tax system is too simple for us!'
Annual tithes from every sentient (modosophont and above) in-system may be employed, but this makes it convenient to appear as non-intelligent as possible. In many systems taxation depends on sentience level:
'Me? Me no super-AI, me be Sam. Sam's just ordinary kitchen appliance. Not pay tax.'
Or consider the tax-dodgers of Eridani, taking on plant DNA to literally become vegetables! Of course there is also the issue of what constitutes a single individual:
'I am sorry, but this company has just a single employee for the moment - me. Yes, yesterday there were 485,499 employees, but as of 12.00 this morning they are me, a group intelligence. This implies that the company should be taxed according to the small company rate, that there is no need to pay social costs and that I as an employee am entitled to tax breaks for my 847,333 dependents and my 2,427,495 kilometer commuting distance.'