I clean my Macbook (same model as yours) by opening it and blowing out the dust, once a year. I use a blower with CO2 cartridge and antistatic brush, always placing the computer on an antistatic mat, with both computer and my wrist attached. I use the Lindy antistatic mat. Keeping the inside clean helps to keep the system cool. If you really want to be thorough you will clean and replace the heatsink paste too.