I think the best way to state it is this: If the native file is close to perfectly sharpened, then any follow-on sharpening is likely to only add detrimental artifacts and not improve the image in any meaningful fashion, so best to not do it.
It used to be I did a capture sharpening in raw, then in post I ran a detail extraction routine to maximize high-frequency detail in the native sized file. Essentially, these two series went after different frequencies of detail. In my current workflow it is this second detail enhancement step that I'm finding is no longer needed. When I downsize for web or small prints, I perform that in CS via "Bicubic Sharper," which in itself contains sharpening as you downsize -- so in that fashion I am sharpening for output, but I do not use any other sharpening tool beyond that. If I have to upsize, my routine uses Bicubic Smoother to 20% over desired size, then Bicubic Sharper to get back down to desired print size. This is sort of a quasi fractal sharpening method, and again is my typical sharpening routine for print and usually no other output sharpening is required. With very large prints, I sometimes invoke a targeted sharpening routine for edges and a second masked set for detail, however, since C1 ver 5 I have yet to need to do this for any of my captures. Finally, if you print via a RIP or printing program like Q-Image, they apply their own proprietary output size-based sharpening algorithms that are usually quite good. (Personally, I send a pre-sized and optimized file to the printer as per above, so I dial this option down to the lower settings.)
As for 3rd party sharpeners like Nik, I honestly have never needed them. Not saying they're bad -- in fact they are probably great for folks that can't sharpen using CS, or who want to paint-in their sharpening. But IMO they're definitely not necessary, and even less so now with the improvements in C1 -- but then I like a simple, streamlined workflow, so the fewer programs I use, the better.
Again, this is just my basic workflow and YMMV...
Good info there. I have saved my sharpening till the image is in LR or PS about to be printed but absolutely need to try C1 for output sharpening. If anything I have a tendancy to under sharpen so a comparison may be hard to prove to even to myself. I normally find the clarity slider draws out most of the needed detail so I then go light with the sharpening.