I saw in the docs that you are not supposed to edit them manually... However is it considered appropriate to do other things such as:
referencing them (adding foreign keys on them)
adding triggers to them (e.g. delete all this stuff if this table is dropped). This would include adding cascade clauses to the referring.
Regards,
Aasmund Midttun Godal
aasmund(at)godal(dot)com - http://www.godal.com/
+47 40 45 20 46