i am currently a pastry student and have learned a thing or two about the creaming method. what i was taught is that the butter MUST be softened to room temperature before anything else (which can prove to take quite a while if you've ever decided to make cookies or a cake on a whim...so plan ahead a little bit). the butter can be microwaved for a few seconds, but i would discourage against that because any butter that has melted is completely useless and absolutely cannot be used in a recipe calling for the creaming method.
the softened butter should be whipped, with either a hand mixer or a stand mixer with the paddle attachment, for about five minutes. it will become light and fluffy before that, but you should keep it going to beat in as much air as possible. then, sugar is added and the mixture should be creamed for an additional 7-10 minutes...YES, 7-10 MINUTES!, again to incorporate air. scrape down the bowl once or twice as well when doing this. the main objective in the creaming method is to create as much air as possible. you will notice the mixture turning a much lighter yellow color. once this happens, scrape down the bowl and the eggs should be added one at a time, waiting until each has been fully incorporated before adding the next, also add any flavorings such as extracts, melted chocolate, etc. next, the dry ingredients are added, or alternated with any other wet ingredients that may be in the recipe. this should be incorporated very gently to avoid the development of gluten, which will make any baked good tough. cake flour has the least amount of protein, so it will produce less gluten. this is why it's the best type of flour you can use for things like cookies and cakes.
i suppose creaming butter and sugar together for such a long time is most important when baking cakes, you must rely solely on the air you produce while making the batter. but, most cookie recipes call for some other type of leavening agent such as baking soda or baking powder, or both. so, i don't think it's extremely necessary to cream for such a long time if leavening agents are called for because they will do their job in the oven. but it also depends on your preference. if you like your cookies "cake like", you definitely want to incorporate as much air as possible. if you prefer cookies that become more crisp, a lot of air will not produce those results. this all has a lot to do with the type of fat you are using too, but that's another post!! anyway, hope you've learned something and i didn't just ramble on and on for nothing!!