Power raking untangles dead grass from the living plants around it, a process that can bruise and even kill the live plants. Power raking should be done either in the spring or fall, when grass is growing vigorously but heat stress is not a problem. This will give plants a chance to recover before they become dormant in winter or midsummer.
Experts say fall is better than spring.
“The increased moisture and cooler temperatures at that time of year cause grass to grow rapidly and recover quickly,” said David J. Robson Extension Educator for the University of Illinois, adding that “Spring is the second best season to take care of thatch.”
If you choose a spring dethatching make sure the ground is at least 55 degrees, the temperature where new grass begins to grow.

Read more: When Should You Power Rake a Lawn? | eHow.com http://www.ehow.com/about_5246762_should-power-rake-lawn_.html#ixzz1dsXQGqvB