In my case, the "fair share" to be paid is decided by taking the decided need elected officials have for revenue and passing that burden on to citizens in a way representing a balance of four interests subjectively deemed to be just: universality (everybody receives benefits, so everybody has a responsibility to pay), marginal utility theory (richer folk have greater capacity to pay tax relative to poorer folk without incurring serious, detrimental affects to their respective qualities of life), minimizing evasion (high rates of taxation, especially on affluent citizens, creates incentive for maneuvers on their part to pay less in tax than what elected officials intended), and economic performance (it is important to mitigate some of the harmful impacts taxes can have in stifling activities which favor human development).
In my opinion, tax evasion constitutes an effort to undermine the first, second, and fourth interests listed and, although not illegal as you pointed out in regards to welfare fraud, is clearly to me a form of individual corruption in passing on the debts one owes to society to others. That is not to say I think individualism ought to be quelled or that it is bad for people to want to receive as much as they can at minimal expense, but an attitude of that sort taken too far in civic life represents a shirking of ones responsibility to contribute to the betterment of society. It's perfectly alright if you disagree, Gustaf, but I am not asking people to volunteer "extra" payments to the state for "fun." I am asking them not to scour the system for imperfections to exploit, and make satisfactory contributions to the state for the good of all.
How do you choose to define someone's fair share to pay?
But presumably the tax laws are written to achieve these things already, no? I can give an example of tax avoidance. My parents run a business from home so they have an office in the house. It used to be that the company owned the house and my parents rented the part they lived in from the company. Then the tax laws changed, making the opposite setup more favourable. So my parents bought the house from their company and rented out the office part to it.
This avoided taxes. But I fail to see what makes it immoral. Of course they could have kept the old arrangement to be nice to the state and kept paying higher taxes than they needed. That would have been exactly the same as paying an extra voluntary payment to the state.
In principle, it's the same as the state cutting the income tax and people then paying the lower rate. Sure, you could say that they are no longer paying their fair share and should continue paying the old rate.