You are asking about ignition timing, right? There is an optimum ignition timing which changes according to engine speed, load, temperature, fuel type, fuel mixture ratio and probably several other factors. Most engines have the ignition timing set a little more retarded than the most efficient setting. This lowers nitrogen oxide emissions, reduces the chance of knocking, reduces the pressure on pistons, rings, and connecting rods, and reduces peak temperatures in the combustion chambers. It is easier on the engine than overly advanced timing. On the other hand, if the timing is too retarded the exhaust valves and valve guides can be damaged because of hotter exhaust gases. If you advance the timing you should do it in small steps, such as 2 degrees at a time. Listen carefully for knocking when you accelerate. Bring along a younger person if you don't hear high pitch sounds well. If you do hear knocking then retard the timing a little until it is gone. If you have high nitrogen oxide emissions during an emission test, that would be another reason to retard the timing.
Here is a nice link about ignition timing:
http://auto.howstuffworks.com/ignition-system1.htm