I couldn't find clarification on this in the manpages and it seems to be a common misconception. Perhaps a note like the following would be beneficial? (Apologies for gmail line mangling)
Index: pfctl.8 =================================================================== RCS file: /cvs/src/sbin/pfctl/pfctl.8,v retrieving revision 1.145 diff -u -p -r1.145 pfctl.8 --- pfctl.8 25 Jun 2010 23:27:47 -0000 1.145 +++ pfctl.8 9 Sep 2010 03:47:17 -0000 @@ -231,6 +231,7 @@ may contain macros, tables, options, and normalization translation, and filtering rules. With the exception of macros and tables, the statements must appear in that order. +It is not necessary to flush existing rules before loading the new ruleset. .It Fl g Include output helpful for debugging. .It Fl h
