I just saw a link to this article on Reddit and thought with the amount of case modding and designing around here, that someone might be interested.
www.pugetsystems.com
TL;DR - A simple wire grill on a fan is best.
Effects of Grill Patterns on Fan Performance/Noise
Fan grills are a component in a computer that is often underestimated in terms of their contribution to both system noise and airflow. Most often, grills are chosen based on aesthetics with only a small thought towards performance. In this article, we will be examining nine different grills to...

TL;DR - A simple wire grill on a fan is best.