as far as i know it doesn't matter.. but there are some cases that the results are different, maybe just glitch.
No, there is no case sensitive matter for Google to place your keywords in the Meta tag. You can write in both the ways and that is considered equally by the Google, so being case sensitive doesn't have any mean. It means only in W3C validation coz that considered only the lower case keywords in the Meta tag.
there have never been any case sensitive keywords in search engine queries, if there is one, it would turn SEO upside down