I noticed a post here about megapixels and publishing photos. The more megapixels the better is a bit of a myth. The main reason being that the CCD on many cameras is roughly the same size so what's happening is that they're cramming more sensors into the same size space resulting in (frequently) poorer sensors.
Not a myth in my experience. I was shooting on a 6mp until the industry standard for pro/prosumer cameras changed. Publications REQUESTED a minimum of 8 mp. I agree that the sensor size and density is mostly irrelevant for magazine publication , but photo editors and publishers want as much resolution as possible. (Yes, it's mostly voodoo on their part). MY EXPERIENCE** is that advertising designers (depending upon the end use, size of the final image) usually want a finished product. For an image that will appear in a low-paying local publication, I usually submit a finished product --- less sharpening. For images that are commissioned for regional, national, and the occasional international publication (and when I am working directly with the publisher) I may submit three versions: Finished .jpg, unfinished .jpg and the raw file, whether I am asked to or not. This allows the photoeditor/designer to take his pick. No matter how determined one may be to calibrate color to a standard, invariably, somebody complains that the colors are off.
At this point, I think that one who sells many images should shoot with a full-frame sensor rather than an APS-C sensor because (as you say) cramming more and more pixels onto a small sensor results in a noisier image. This can be overcome, but if one shoots stock, one usually doesn't have the luxury of putting his image through Noise Ninja and other such programs. A full-frame sensor image shot at 25 mp is a far cry from anything shot on an APS-C sensor , then output at 8"x10".
Re: Postprocessing. For stock photography, the larger the file the better ( Assuming that you submit your file batches as .jpg files). The agencies that handle and have handled my images want NO PP whatsoever, not even sharpening. That's why your advice regarding lens quality is so pertinent. Generally, sharpening is the last thing that should be adjusted in an image file. Start off with a great image, then let the end user manipulate it to his heart's content.
** Your experience may differ.