"Category:Born in 1790" should be unambiguous and self-explanatory. However, somewhere in our documentation is the recommendation that people should go in that category if their birth was only known to be "about 1790" (or "c1790" in our recommended form). User:Rtol, after discussion on his talk page, has devised a template that will do that for any year. Excellent (I expect). See Template:B-c. Doubtless D-c and M-c will follow.
Next step in the logic, however, brings us to people looking for that "c1790" individual from the starting-point of a different "known fact", such as "Born before 1798". How many years should that searcher check? Should we be using "Born in the 1790s" for all such individuals or would that just make one more category to be searched? Newcomers may not even think to look in "Born in 1790" because, as I said at the beginning of this essay, that category name seems self-explanatory.
One solution could be a belt-and-braces approach whereby anyone born in "c1792" or "c1793" or any time up to "c1798" would also go into the 1790s category. For 1789, 1790, and 1791, I suggest that they go into that decade category and the preceding one, the 1780s. User:Rtol and I believe that we personally are not quite up to creating a template that deals with "ifs and buts" so as to produce one decade for one year but two decades for another year.
Anyone feel like tackling that one? (There are some templates already here, relating to decades, that do that sort of thing by splitting off the last digit, eg |179|4. That could do one decade automatically. Tricky bit would be to do two.)
There are four cases to be considered: c1790 (around 1790), 1790? (probably 1790), aft1790 (surely after 1790) and bef1790 (surely before 1790) Rtol 13:31, 22 February 2009 (UTC)