Lot of adventurers have lots of gold. Something that any right-thinking ruler will want to get a percentage of. However, taxing a bunch of even mid-level adventurers could cause untold destruction if they are of evil bent. Also, what percentage, exactly, should be taken from the loot gained by tomb-robbing (which much adventuring really ends up being). And how much goes to the state, and how much to the church? Since both like to get their own free money. I suspect tax collectors will need to have magical backup, or at least a squad of toughs, for when they collect the dues.
In addition, some of the taxes might be "paid" in service, instead of with gold. This could provide the occasional plot hook, as the adventuring party is required, by the local baron, to get rid of a bandit holdout. This service will be done at the adventurer's expense, but will count as a tax payment. This way, the baron can get some criminals taken care of for free, or some annoying adventurers might get killed off and their remaining properties could be taxed or even confiscated. This will be a win-win for the baron. It's also a fun way to get players involved in the local setting, and worried about politics.