While we certainly do not want to discourage a site owner from adding functionality to their sites by adding javascript.  There are some things to take into consideration.

1.  It's best to discuss the addition of any javascript to your site with the webteam.  
2.  If you add it, it's your responsibility to make sure it works for everyone who accesses it.  And to also make sure it does not break existing Javascript.
3. You'll need to test it in all typical browsers and on both Mac and PC.   That means on a PC; IE6, IE7, IE8, Firefox 3x, Chrome, and Safari (most recent version on both the last two).  On a Mac that means Firefox 3x, and Safari.  
4. If you add javascript, it could break existing Javascript, so double check this by also checking the functionality of any original javascript (if any) when you test. 
5. Do you know about using/coding Javascript?  Can you support this javascript?  If we didn't write the javascript, we can't support it.
6. The script you add cannot cause any javascript errors.  This means that it needs to play nicely with both IE6 and IE8. 
7. Please stop and think before you add it.  Is it functionality or bling?  Is there a solid reason for needing it?  There is a tendency to want to make a page lots more fun by adding lots of neat stuff...  which is great for a personal site or for a site that is purely for entertainment.   It could truly hinder or push users away from an informational site.
8. The page you've added it to must still validate  (please see Web Accessibility Policy and Standards for more).