I think your last question may be better handled later in a separate Q&A but I'll try and cover off on the first four here.
- I think the event tags like world-war-1, 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 19th-century being used to the exclusion of decade tags like 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]
[1920s] or [1930s]
in the search bar - search using 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]
[1940s] or [1930s] or [1920s] or [1910s] or [1970s] or [1960s]
I would not like to see more than two tags on a question devoted to its date information, preferably only one (and none sometimes), so that there are enough tags left for one or two to summarize its geographic location and a number of other keywords.