No, it is not OK to punch Nazis
The reason violence it is not OK is because we have a social contract that helps preserve order.
While people sharing "punch a Nazi" memes on social media may think it is "cute" or funny, it is actually not appropriate to use violence against someone for his political opinions. That is a universal rule and applies to everyone, no matter how perverted, depraved or downright evil their opinions may be. So, no, it is not OK to punch Nazis and it never was OK to punch Nazis. You are not allowed to punch Communists either, and both the Soviet Union and Red China murdered more people than Nazi Germany.
I cannot believe I have to explain this. The reason violence it is not OK is because we have a social contract that helps preserve order. Once you say it is OK to punch a Nazi, then what about a member of the Ku Klux Klan? What about someone who is simply a racist? What about someone who supports - or opposes - legalized abortion? What about someone who supports - or opposes - government recognizing homosexual marriage? Moving away from hot button issues, what about people who support restrictive zoning laws? Should I be allowed to punch someone who restricts what I can do with my own property?
You say, "Well, Scott, that is absurd. We're not talking about land-use policy, tax policy, or even a hotly contested social issue." But it is actually not absurd. Once you establish the principle that you can use violence against someone because of his political opinions, the question ceases to be about whether violence is appropriate. The question then becomes where you draw the line for what political opinions can subject someone to violence. Can you see how that viewpoint can be incredibly dangerous?
Once you establish that you can punch someone for his political opinions, then it is a short step to open gang warfare in the streets between rival political factions. That is the kind of thing we see in fledgling democracies in third world nations, or when tyrannical regimes are collapsing and the order imposed from the top no longer restrains factions. Furthermore, if you think that "punching" will be confined to adults, you need to study history. Children will be targets of political violence too, along with women and the elderly.
The solution to an evil ideology like Nazism is not punching someone in the face. That does not convince anyone, it makes you look like an unhinged lunatic, and it creates sympathy for wicked people who deserve none. The answer is to expose just how wicked and evil Nazi ideology is, and to work to convert Nazis to a better worldview. Those who engage in violence are not courageous. They are lazy and they do not have the intelligence to defeat wicked ideas. They are also almost always cowards.