Grab (the OSX screen capture utility) creates TIFFs by default, and there is no way to change that (The settings dialog does not offer a file format option, and I didn't find any other way). It does so irrespective of the default screen capture format (which is PNG unless you have a very old OS X version).
That is, even if you use the defaults write "hack" as described above, Grab still creates TIFFs. If I recall it correctly, previous versions of Grab at least allowed you to change the format in the settings. It is beyond me why Apple chose to remove this setting, and default to TIFF, as TIFF is a format that is used mainly by image editing professionals and in prepress applications, it is wasteful in terms of file size, and most browsers don't support it natively.
Converting the image using Preview or other tools (even Automator actions) is not practical, it's much too cumbersome and takes too long.
The solution: don't use Grab.
I recommend to use the keyboard shortcuts as suggested above, which is by far the fastest way, once you know them by heart.
The alternatives would be to use dedicated and more powerful screen capture utilities such as Skitch, which also include annotation tools so you can highlight and label certain parts of the screenshot.
