Emphasis on the "properly". :-) I had been going with the conventional wisdom
of sites/<site>/files, but when I went to merge two sites into a multi-site
to deploy them realized that wouldn't work; the path that is stored into the
files table is then bound to the domain the site is on, which is no good
because the domain changes from staging to production. So if we document
sites/default/files for single-site installs, we need to in the same breath
document to *not* use the sites/ directory for multi-site installs unless you
want to make moving to a new server to be a PITA.