If you are in Windows you can use a DOS command prompt (Run...>cmd.exe) after you render the tif files WITH the extension in place and then type within the render directory:
" REN *.tif *. " (leave out the quote marks). You can do something similar in Mac OS X with the terminal, but you will have to search the internet for UNIX/Linux commands to rename files.
. . . . what the other two said above . . .
+ there are plenty of free file renaming applications on the internet - just do as has already been said and render WITH the extension - then simply remove it from all the files with a file renaming application - they are very easy to use and fast (a couple of seconds to rename a thousand files).