The manual for the 7D is a little vague about the topic, but here it is:
Sometimes the vibrations caused by the mirror flipping up before exposure can cause movements in the camera that can blur the image. It usually happens when using shutter times around 1/20th of a second, with long focal lengths on a tripod (such as macro photo).
Shorter times are "quicker" than the shake movements, and in longer times, the shakes will have "faded out" for most of the exposure. The AS in the 7D should effectively counteract this, but it never hurts to be safe.
Using mirror lock-up on the 7D is really simple: Use the 2s self-timer mode. The mirror will flip up as you press the shutter (with an audible click sound), then it will stay up the 2 seconds until exposure thereby allowing the vibrations to die out.RAW and RAW+
If you want to shoot in RAW format, consider using the RAW+JPEG (RAW+) mode. If you want to save memory space, use "Small" (1504x1000) and Standard quality. The JPEG file will be about 400Kb average (the manual says 540Kb, but it's usually smaller than that) which isn't very much compared to the 50 times larger RAW file. Using default (+/-0) "Digital effect" settings, sRGB (natural) color space and automatic white balance (AWB) usually work. The main benefits of this are:
[*]The ability to zoom the image shown in the display of the camera
[*]Easier and quicker previewing of files when looking through them on your computer
[*]A decent file size for instant web publication, with good enough quality for "everyday use" without having to go through RAW conversion
For the few good images you might want to spend the time to perfect, you have the maximum quality of the RAW file as well (with the ability to set white balance and other settings in "post-production").