Baa would likely be the sound that sheep make. Trust me; I'm Welsh.
I would say these posters are lamenting the fact that people believe they are somehow made superior by 'rebelling', when in fact, their act of rebellion against one stereotype results in them fulfilling another.
Say you were a teenager. And say you weren't accepted by 'cool' people, in the 'popular' sense. And say you fell in with another group, where people talked about 'rebelling', and stood around in 'Slipknot' hooded sweaters. Whilst they may tell themselves that they are rebelling against the 'cool' people (Who wouldn't accept them) by not conforming to their standards, they are in fact conforming, just to another set of standards (Those of the group that was willing to accept them).
They they tell themselves they are rebelling against the popular culture of the 'cool' people, but some would say that they are simply telling themselves this, as the 'cool' people were not accepting of them.
So, yes, I would say that the poster qousquos speaks of was saying that people who rebel by buying mass-market consumer products and who call other people sheep whilst conforming to other stereotypes themselves are, in fact, as much 'sheep' as the people they claim to be rebelling against.
What do I think? I don't think the opposite of conformity is rebellion. The opposite of conformity is independent behaviour, regardless of social influences; True independence is acting on one's conscience, making reasoned decisions rather than being explicitly conformist or non-conformist.
An example would be Galileo's resistance to ideas of his time; this was healthy independent thought. Conversely, going around a roundabout the wrong way would be foolish, not independent.
I would say someone is a 'sheep' if they conform to standards not for a considered reason, but because they are 'following the herd'.
You can't talk about the sheep who still run Windows, or classify any other group as 'sheep', for the simple reason that they may well have considered their actions, and there is likely a solid reason behind them.
For example, I use Windows over Linux because I do not need the additional functionality provided by linux (Network scanning tools, multiple user support), but I do need the additional functionality afforded to me by a Windows computer (AutoCAD 14, 3D Studio Max, Microsoft Office). I have looked at the availiable options and made a reasoned choice. I am not, in this respect, a sheep.
Anyway, here's my point: Not wearing (or wearing) a Marylin Manson shirt doesn't make you a 'sheep'. So quit calling other people sheep.