I would say that blown highlights or blocked blacks are perfectly acceptable if it helps you meet your artistic goals! There are different and often conflicting aspects of photography that sometimes make it difficult to judge this kind of thing. On the one hand, you have the "technicals", the aspects of mechanics and technology, telling you not to blow out your highlights. Its "bad" because they can't be recovered with digital, and there is a bit of a taboo about it. On the other hand, you have the "art", the aspects of vision and style that make a photograph all you. At times, the artistic desires of your minds eye are at conflict with the demands of technology.

When push comes to shove, I would say go with your artistic side. If you visualize something that moves you, capture it how you see it. If that means blocked blacks or blown highlights, including a bit of sun flare, allowing some vignetting, then so be it. Ultimately, art is what photography is all about...not meeting some logical definition of what photography should be from a technical standpoint. Use the tools at your disposal to expose your vision, and push the limits of those tools to bring about your own personal style.