You should only dethatch when it is needed. The dethatching process causes a lot of damage to the plants. Verticutting is actually better. That is using fixed knifes to slice through to the surface. but back to dethatching. If the thatch is more than a half inch thick in your area it should be removed. Less than half inch you can do the lawn more good by aeration. Actually aeration will help on all lawns but remove the thatch first.