dstiles - 1:36 am on Mar 26, 2010 (gmt 0)
I think it depends on whether the url is complete in itself or is the root of a default index file. Eg example.com/ is expected to load the home page, and example.com/folder/ would load the default page in the folder.
As I recall, the web browser, if it fails to load the default page from (eg) example.com/folder will add the trailing slash and try again (not sure if it's the browser that does this or if the server issues a redirect; I think the browser, from memory). So in that case it makes sense to include the trailing slash to avoid extra work.
If the url example.com/pagename is an actual page (ie a file with no extension or an interpreted name) then the url is complete in itself and should not be expected to have the trailing slash which would, if present and unchecked for, cause a 404.