WHY would you add a slash to a valid URI? That tells the server that it should look for a directory, not a file. (IMHO, it's a bad technique that some webmasters are trying without understanding the adverse consequences.)
DON'T add a trailing / unless you're requesting a directory (in which case, servers will usually add the / for you). Remember, you're affecting all the relative links, too, by changing the directory level like this.