Pick a few faces and address them, slowly switching from one to the next. Choosing them from the back of the audience is good advice, too.

probably the most important issue is timing. If you have practiced and know that your timing is right, there are really no reasons for worry. But if you become afraid that you won't be able to finish in the alotted time, or, if you finish too early and there is an awkward pause to fill, then those moments would be anxiety-ridden.