I think your last question may be better handled later in a separate Q&A.

 - I think the event tags like [tag:world-war-1], [tag:us-civil-war], etc work well and if I was running out of tags on a question I would have no qualms about including no other date tags
 - I used to be a proponent of century tags like [tag:19th-century] being used to the exclusion of decade tags like [tag:1920s] but I have come to thinking that decade tags can be very useful.  Personally, I would now be in favour of questions having:
  - one or two decade tags and no century tag
  - one century tag only when the question concerns three or more decades

As an example of using a tagging scheme like the above where I might be looking for something that happened ***between*** the two world wars I would:

 - not search on the world war tags
 - search using [`[1920s] or [1930s]`][1] in the search bar
 - search using [tag:20th-century] (knowing that I might get many questions that are outside the two decades of interest but to be sure that I see some that might not have decade tags on them).

One useful search to be aware of is that if you type `[19*0s]` into the search bar it will expand to [`[1940s] or [1930s] or [1920s] or [1910s] or [1970s] or [1960s]`][2]

I would not like to see more than two tags on a question devoted to its date information, so that there are enough tags left for one or two to summarize its geographic location and another 1-3 keywords.

  [1]: http://genealogy.stackexchange.com/questions/tagged/1920s+or+1930s
  [2]: http://genealogy.stackexchange.com/questions/tagged/1940s+or+1930s+or+1920s+or+1910s+or+1970s+or+1960s