What Would Jesus Type?
I know there are some questions about what “rules” members should use for tags and concerns about consistency and accuracy, etc.
I don't presume to speak for SO on this, but here's my take based on my work with various kinds of database systems:
Don't worry about it.
It'll be fine. Search systems like this actually work better if members “widen the field” by being creative with their tags.
If members are too rigid in categorizing threads, the benefits of member tagging are actually diminished.
As an example, say you use the same keyword to describe all threads on Iraq: the word “Iraq”. There are thousands of threads on Iraq, so if you
search for the keyword “Iraq”, you'll get thousands of hits.
Helpful, but not very.
Celebrate Obversity
But if you get more specific -- and creative -- and offer more specific terms, like “Battle of Fallujah”, “Iraq Constitution”, “Zarqawi”
or “Butcher of Baghdad”, then someone searching for that data will definitely appreciate the more specific tag.
On a grand scale, the general idea is that members will type in the kinds of words/phrases to tag a thread that they might use to search for the same
thread later. Or so the theory goes.
Thus a good way to think when you're tagging a thread is “what would I type to find this thread?”
Or not. Your call.
It's all good, baby.

Edit: As for case sensitivity, I would be extremely surprised if that were an issue. For some funky reason, I've been capitalizing my keywords (maybe
I subconsciously think of them as subtitles), but that's an unnecessary affectation.
Case-sensitivity is generally anathema to search systems, so I don't see the need to worry about it, and I'll probably just use lowercase going
forward.
Put another way: if it mattered, I'm sure the Overlord would have mentioned it.
[edit on 12/15/2005 by Majic]