Historically, income tax was not in fact a given. In Canada, it was only leveled in response to WWI, in fact. Republican rome dropped its taxes on citizens in rome proper in 167.
While they retained taxes (on assessed wealth, not income as they did not even try to track such a thing back then) in the provinces, the difficulties of performing a proper census on the population led to tax assessments being made against entire communities instead of individuals, with local authorities being responsible for figuring out who would pay what.
In the medieval period, land taxes were more often leveled than income taxes. Tariffs on trade goods in transport were also somewhat common. Income taxes (in general) were not common, or particularly popular. They also would have varied from location to location, as generally a King would tax his barons, and the baron his vassals, etc. and local lords would pay their share out of crops grown on their own lands using labour provided by serfs (as a certain amount of labour on the lord's land was generally owed to the lord by his serfs).
User fees such a bridge tolls and the like were also a common source of revenue for governments at the time.
Without a central tax agency or accurate censuses, taxation of adventurers would be more ad hoc.
Basically, if rumours reached a lord with local authority who thought he was powerful enough to get a share, he would have to put together an armed force.
That force would then make an expedition to assess the wealth of the party (which would likely just be the opinion of the leader of that force) and then demand some percentage of that amount at sword point (1/15 of moveable wealth was fairly common) and whatever land tax is proper for the amount of land they own. (2 shillings/hide of land in Henry II's rein, for instance). Even then, if none of them were sworn to that lord as a vassal or serf, the legality of the taxation might be in doubt in the first place.
In order for that lord to hear about it, and then spend money on a military adventure of this sort, the amount of money would have to be rather large. As well, the lord involved would have to assume that the adventurers should be treated as vassals/serfs. It is just as likely that he would consider them to be thieves or bandits and their goods to be forfeit if he won the day.
That is also excluding any items which the king or lord of an area might have proclaimed as expressly his wherever they are found. In England, for instance, the head of a whale that beaches itself belongs to the king, the tail to the queen.
That also excludes things like the tithe (10% of earned income goes to the church) which were solely collected based on the risk of excommunication that the medieval church had.
While in the fantasy society you are running a game or, or playing in, might be more anachronistic about taxes, in real history, they were not so inevitable or unavoidable as today. Information was hard to collect, and when collected, was not centralized. No one was reporting what they paid a worker to the government. Every tax bill would require boots on the ground to access an amount, and hiding wealth was common. Usually by burying treasure somewhere. I think it would be safe to assume that taxes aren't a worry for PC's, unless you think a visit from the sheriff would make a good story.