With gamut compression set at 0% or off, the display profile will behave like the Relative Colorimetric rendering intent of a printer profile. In-gamut colors will be as accurate as possible, but out of gamut colors will be flattened at the edge of the gamut. This will result in a loss of detail if you are working with images that contain colors that your monitor cannot reproduce. Increasing the setting causes more compression to be applied to the profile, like the Perceptual rendering intent of a printer profile. This will allow you to perceive more detail and difference among out of gamut colors, but will also compress in-gamut colors, making them less colorimetrically accurate.
The default setting of 60% is a good starting point for users softproofing on an LCD monitor. In many cases, a colorimetrically accurate rendering of an image will still appear too saturated when viewed along side a proof. The "best" setting will depend on all the criteria listed above. However, there is a convenient way to create multiple profiles with different settings. The "Preferences" menu item is available when you are at the "Save Profile" step. Rather than remeasure the entire patch set for several settings, you can adjust the Compression setting and save multiple profiles, each with a different setting. You can then evaluate the profiles later by switching between them to determine the setting that best suits your needs.