default select ordering

Please accept my apologies if this is answered elsewhere in the archives
or docs but I have searched without luck.

I've always assumed that default ordering of selects are based on a first
in first out principle and that this remains true at the row level despite
edits to columns.

However I'm dealing with a case where this doesn't seem to hold true. The
ordering has changed over time. Its difficult for me to gauge whether the
data has been moved around with pg_dump or whether optimisation/vacuum
commands have been called but I'm wondering if either of these things
could effect the first in first out ordering and if there's any system
tables or configerations that I can use to either verify or prevent this.

Any help greaty appreciated.

Matt Roberts