Jürgen Hubert said:And in my opinion, that's not a bad thing. For this will get even the most politicially ignorant PCs involved in politics, which gives the evil DM all sorts of plot possibilities...
With such an action, the PCs will make a powerful statement that they will not respect the authority of the local ruler. This might cause retailation from said ruler, who does not want to see his authority erode, rivals of said rulers, who wish to see his authority eroded further and want to use the PCs to that end, general resentment from "respectable citizens", who don't see why they should have to keep paying taxes when those thugs don't, and so forth...
Forgive me for saying so, but it sounds like you've already made up your mind.
Anyway, as others have pointed out, it comes down to the ressources the ruler has to enforce his will. A group of soldiers might be able to convince a low-level adventuring group to let go of part or all of their newly-acquired wealth, but that will become harder at high levels, unless the ruler has (presumably) loyal squads of high-level enforcers at his disposal.
One thing I think I'd do would be to be upfront about the local realm's loot taxation laws. If the party knows going into the dungeon that the law dictates a fifth of all earnings going to the crown, they won't have reason to complain when the tax collectors show up when they return in town with the hobgoblins' treasure hoard. On the other hand, if the adventurers emerge triumphant from the lair of the red dragon that's been terrorizing the kingdom for generations, only to find some bespectacled twit and a group of soldiers demanding they hand over all the treasure in the name of the king when they've never heard of any such law, they'll be naturally inclined to demonstrate why they were able to kill the dragon in the first place.
Another valid concern is that taxation might make the PCs underpowerd for their level. "Right, I'd love to be able to hit that pit fiend, but after taxes I didn't have enough gold left to sufficiently enchant my sword. Joy." Perhaps you could give them extra loot over recommended wealth levels, with the expectation that the extra loot will soon be lost from taxes.
There's also the issue of reasonability at hand. How much can people who have risked life and limb for their gains be expected to hand over in taxation before deciding taxes are too high and go see if the next realm over is less greedy, and to hell with that upcoming orc invasion? The amount might well vary from party to party, but if a realm consistently shows itself over decades and centuries to be rapacious where adventurers' earnings are concerned, word will logically spread throughout the adventuring community.
I agree that loot taxation can provide a lot of opportunities for roleplaying, but IMHO it also needs to be handled carefully, as it could easily turn the PCs off the realm where it's happening, or even the players against the DM.