Agreed - I've seen it cause duplicate content issues with URLs, but I'm not aware of situations where it mattered in on-page keywords. I don't like lower-case page TITLEs, because they look spammy, but that's more a CRO factor.

One minor exception, but I'm not sure it's in play anymore. I once ran into something where acronyms were pulling up different results depending on what case they were in. Google might have been mistaking the acronym in question for a misspelled word, though - it's a really isolated case.

Someone who will not be named (it rhymes with "Bomb Itch Slow") told me to Mechanical Turk the crap out of it

I have some smaller clients who have had limited luck with it, but I think it's best to just stick to one of the sites and do periodic releases (maybe every couple of months). If nothing else, it'll give you a sense of what's working, and you can take some of the popular releases and push to get them a broader market.

What I wouldn't do is go after multiple low-value PR services and plaster the same releases everywhere you can. At best, it's diminishing returns - at worst, they'll be devalued. I'm with Peter G. - the best press release opportunities come through relationships with the media. Obviously, though, that takes a lot more time.