what do u have done to your ls motor? if its stock you shouldnt mess with the timing and leave it to the factory settings. People usually advanced the timing on their motors if they use a different octane rating of fuel. Low octane fuel will require you to retard the timing and high octane fuel will require you to advance your timing. Also the heat range of your spark plug can effect what ignition timing can be used. Another reason is if you change the rpm limit of your motor, then you would have to change your timing. Baseically advancing and retarding the timing is mostly used b/c of abnormal combustion such as your motor knocking or having preignition. In abnormal combustion, the air, something in the combustion chamber or even the whole chamber is too hot for the fuel or it is compressed too high which itself causes the temperature to raise too high, causing the fuel to explode. This explosion produces extremely high temperature in a certain spot and causes a pressure spike so high and quick that it is like a hammer strike. So in plain english, if your motor is stock then dont advance your timing unless you hear a knocking or pinging sound.