I was renaming a bunch of files on our picture file server and was tired of getting a confirmation window everytime I wanted to add .tif to a file. I looked in the preference for Finder and found something on the advanced tab. If you place a checkmark in the "Show all file extensions" checkbox, you no longer get a window asking if you're sure you want to rename each file when adding an extension.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031230172808219