since you're now looking for heuristics, and nobody has yet
mentioned working with multiple columns, let me address that a bit
from experience here at ams, mostly with the ams notices.
if there is a running footer, that, of course, always goes at the
if there is a spanning bottom float, that goes next at the bottom;
one-column footnotes go above a spanning float. i have no experience
with spanning footnotes. however, there are a number of situations
where ancillary material is packaged into a spanning bottom float
with its own multi-column arrangement; the number of columns in the
float may, but need not, be the same as the number of columns on
the main part of the page.
a spanning bottom float which covers fewer columns than the whole
page goes above any full-width spanning floats. one-column footnotes
are still above this.
a one-column bottom float goes above footnotes.
a footnote goes at the bottom of the same column as its callout.
unnumbered footnotes (first-page credits, e.g.) always go in the
column in which the item begins.
if the last column ends "short", any footnotes in that column
still go at the very bottom of the page.
a one-column, full-page-length float appears to be preferentially
placed in an "outside" column.
tables are more often placed at the top rather than at the bottom.
footnotes to a table appear directly below the table.