Cheating on taxes is a complex subject (perhaps its complications lie in the negius of the people making arguments).
The side SJS is presenting is that by cheating on taxes, the government receives less money and must make up the money from other tax payers (since their spending is not a function of revenue collected but the converse). Either the current tax payers will pay more now or the missing amount adds to national debt and will be payed for by future taxpayers. The tax evader is thus stealing either from his own cohort of taxpayers or from future generations of taxpayers.
The other side of the debate seems to be the argument that it is not stealing because the government always collects the maximum amount of taxes it can safely demand at any time. No one is paying more as a result of your cheating because they are already paying as much as the government could get out of them anyway. I’m not aware of any other defenses, and this one sounds pretty flimsy and specious.
Of course, there are the anarchists who claim that taxes in and of themselves are theft and therefore it is not theft to avoid paying them (any more than it is theft to lie to a burglar and say there is no jewelry hidden in your home when there really is).
If there are any real anarchists among us I would be surprised (it is not a Torah concept, for one thing). As far as other arguments go, it always comes down to something that sounds like excuses. I’m with SJS all the way on this one. If you cheat on taxes, you are reducing the amount of money in the US coffers and adding to the national debt. That amounts to thievery in my books.