I think this thread and the general consensus is that morality plays no role in paying taxes. The only people I hear that use morality to pay for taxes are people who talk about how immoral it is for billionaires to pay so little in taxes. I would argue that that is a poor way of talking about the problem. In pretty much every way I know of, the world immoral means it's immoral to do something regardless of what others do. I would bet money that people that so frivolously throw around the word immoral for billionaires use whatever tax loop holes or means to reduce their taxes that they can. They would use excuses like, "I'm not making that much" or "I deserve it" or "I'm doing other things to help out the world that those billionaires are not."
The point is, everyone, if they think of it as a moral issue at all, thinks they're doing good stuff that merits them take tax breaks and the like. Billionaires likely think the same. In the case of the Gates and some others, if they truly give away their entire fortunes (or 99.9% of them or whatever), they might actually be right.
If paying less taxes than you "should" was a moral issue, people that don't pay more in taxes are almost certainly hypocrites. However, it is a completely legitimate position that overall, the system can be reformed, and I'll vote for people that will make me pay more in taxes and everyone else, as well. If you address it as a moral issue, however, then what's wrong is wrong, and you almost certainly should be paying more in taxes yourself.