We almost always recommend selecting a target gamma of 2.2 when your profile your display. Most monitors have a native gamma that is close to 2.2, so by using the 2.2 setting you will get a more linear and consistent result in your soft proofing.

Apple monitors at one time standarized on a gamma of 1.8 because it produced a closer preview to the Apple Laser Writers. However, Apple now recommends a gamma of 2.2 for their LCD displays.

You can always test the optimum setting for your soft proofing workflow by creating monitor profiles with target gammas of 1.8 and 2.2. Compare to see which setting gives you the best results for your workflow.