JS_Harris - 9:04 am on Mar 4, 2010 (gmt 0)
The only notifications you need are those sent directly to search engines. Since search engines also keep a close eye on services like Twitter it helps to send the content to them as well (feedburner has an auto twitter snippet posting service for wordpress).
I like to use the other services as intended, like Google reader, by letting them know I exist and letting them visit when they see fit. If every blog out there pings everything it possibly can a huge amount of noise is created that gets ignored because search crawlers are already working their own schedules.