IMHO, If you do not want it indexed, block it via robots.txt, or via a noindex in the doc. Leave nothing to chance, if you do not want them to index something, tell them.
Edited to add, I personally do not block my 404 custom pages, I do not link to them, but I figure, if a search engine finds it by chance (and they do), and happen use whatever content I put on it, and to place some sort of importance on it, and drives a bit of traffic to it, so be it.
Edited 1 time(s). Last edit at 06/24/2008 08:22PM by wrench.