Well, if you are asking in terms of taxation then is most of the countries, you will be taxed only when you will cash out your funds and hence if you don't want to cash out a certain portion of your earnings then it will remain untaxable as far as you are not cashing out those funds. On the other hand, if you are earning a living from your freelancing work then you will have to pay taxes since you will be cashing our the funds in that case so considering your situation, you can decide that if it is appropriate to charge fees in BTC or not.