Good point. You can do it globally or on a forum-by-forum basis. For example, both my Announcements and Tutorial forums, which I'm working on for vB 3.6, are located within the forum directory. Using the rewrite rule I changed it from:
www.mydomain.com/forum/announcements/
www.mydomain.com/forum/tutorials/
To:
www.mydomain.com/announcements/
www.mydomain.com/tutorials/
Having the forum in a separate directory keep things clean.