Depends on your lighting and aperture and desired shutter speed (same as with all photography). I recommend reading up the newbie stickies in the macro section, as well as the stickies here in the newbies corner. Specifically, learn about exposure.
The general guideline would be.. "As much as possible, you would want to use the lowest possible ISO"
But before that. You will need to ensure that ISO can give you the shutter speed you want or is sufficient enough for your intended purpose.
Different situation calls for different setting. Also depend on your set up.