Do you know if leaving the sharpness setting to zero makes the image worse or better. I find that putting it to 7 makes it "sharp" but something seems artificial with it when I review the images. Does this make a difference if you're shooting RAW anyways?
I have a 24-105 L and usually shoot it at f/4.0 and around 24mm.
Is there a sharpness setting that works best for anyone?
The sharpness setting does not make a difference to the RAW image, but would be applied if you're shooting JPG.
I don't know what setting would be best, but if the range is 0-7 I would think an intermediate value (3 or 4) would be a good starting point.
If you are going to err, I'd err on the side of having less sharpening, because you can always sharpen in post processing.
@MikeW has answered the question about the effect on RAW, but to answer the other half of the question: changing the sharpness setting doesn't make the (JPEG) output better or worse: it makes it different. Increasing sharpness generally leads to better edge definition, but also to a greater visual appearance of noise and additional artifacts caused by the sharpening algorithm. What's "best" for a given photo depends both on the photo and on personal taste.
Unsurprisingly, there isn't a setting that is best for everyone. If so, you wouldn't get the option to change it :-)
If you read the 7D Review at The-Digital-Picture you see that Bryan Carnathan makes quite a few references to the need to increase the sharpness setting to "2" or "3" to match the output of many of Canon's other models (at the time the review was written). He attributes it to a stronger anti-alias filter. This makes sense as the 4.3µ pixel pitch on the 7D's sensor is tighter, and thus a higher frequency, than the other models he compares it to.
When shooting with my 7D I normally select the "Standard" picture style which includes a sharpness setting of "3". After importing the RAW file to Digital Photo Professional (DPP) this setting is used to initially display the photo. In general, I sharpen using the 'Unsharpen Mask' tool which replaces the in camera sharpening setting with the parameters I choose when the file is converted to JPEG or TIFF. Of course this is done non-destructively by adding a set of instructions to the file. The original setting is still attached to the file as well and can be restored by clicking the Return arrow to the right of the Picture Style heading on the tool palette (Even if you open a copy of the file made after you changed the settings later on a different computer).
Your final sharpen settings should be determined by the display size you intend for the image.
See also Recommended start point for sharpening RAW Images. There are a lot of variables that will ultimately determine your sharpening settings. The good news is that if you save your images as RAW files, you can always change it later if you want to.
Very simply, unless you like spending time doing things the camera can do, afterwards, set a mid value, and try test shots at other settings to see your preference. I often find professional photos that need a bit more sharpening and a good bit more contrast. The camera can do this for you and give you clearer more vivid photos. Why waste your time afterwards?