In my recently adopted state of Minnesota, we (the SHPO
archaeologists, the state archaeologist, and interested
historical archaeologists working in the state) have decided to
issue site number to late 19th/early 20th century sites only
under the following conditions:
 
1.  the site has been the subject of professional
archaeological investigation sufficient to characterize the
deposit as actually a site (as opposed to field scatter
originating in a manure pile, or
 
2. the site has been determined eligible for the National
Register (may or may not have included substantial field
investigation), or
 
3. dates before c. 1860 (i.e. around the time of statehood).
 
The rational for this policy is to make sure that only "real"
sites get into the site inventory.  It is much more difficult
to get sites off once a number has been designated (although I
don't really understand the reason for this).
 
Much of the upper midwest is trying to make sense of late
19th/early 20th century sites, especially farmsteads.  The how
and why of their significance has not been well established.
South Dakota, however, did a historic context for farmsteads
that has been helpful to folks in nearby states as well.
 
I would welcome others thoughts on this topic, especially since
I am preparing a paper for the Midwest Archaeological
Conference on this particular issue.
 
Thanks,
John P. McCarthy
Senior Archaeologist/Historian
Institute for Minnesota Archaeology/
IMA Consulting, Inc.