You can take a "plug" sample from different places in your yard and look to see how much thatch build up you have. If you have less than 1/2 of an inch then I wouldn't worry about using a power rake but if you have a good 1/2 inch or more then it would be a great idea to aerate then power rake. But before you do those two things you should mow your lawn the lowest you can for your grass type, so not to harm your grass and keep from scalping your lawn. Also make sure you bag the clippings when you do this. After mowing, aerating, and power raking go back over you lawn with your mower with a collection system and pick up the thatch and the plugs created from the aeration and power raking.