octane is the fuels burn speed rating. Higher octane burns slower, that is why you would use higher octane in a high compression motor, or boosted application. The slower burn speed curbs off detonation allowing you to run higher boost, or compression. As the others said only buy the grade of fuel needed to keep the engine from pinging under a load. If you can accomplish that with 87 octane than 100 octane would probably show a loss of performance, without bumping the timing up to accommodate for the slower burning fuel.
So then your left with turning the timing back to what it was if you had to fill up somewhere else sometime.