I would agree that repairing permissions before the update may be a waste of time, especially under Leopard, notoriously slow in this respect. We had extensive coverage of the database glitches causing tens of permissions repaired every time. However, I would disagree that repair is not recommended after the update. When I went from 10.5.6 to 10.5.7, the usual permissions were "repaired". At the same time, transition to 10.5.8 with a combo updater caused a much more extensive "repair" (hope this is corroborated by others). I suspect it was prudent to run DU after the 10.5.8 update, although the meaning of this repair is kind of obscure yet. Hopefully, Snow Leopard would take care of the database issue. But I agree 100% with the last paragraph in Doni's post above.
Update: MacFixit article highlights real problems with permissions related to a new database glitch in 10.5.8, and provides a Terminal-based cure:
http://www.macfixit.com/article.php?story=20090806095905196
