I worked for the local Stihl dealer for a while running his shop. All the newer saws spec 89 octane per the manual and the older saws spec'd 87. There was a service bulletin come across the bench that due to the addition of alcohol to 87 octane all equipment should be upgraded to 89 as it is not required to contain any alcohol and usually does not.
If you want a complete technical paper on alcohol and small carbs you should toddle over to the Walbro website and read the tech papers there. It is very enlightening reading and can even cure insomnia for those that don't really understand the basics of fuel induction.
You will be glad to pay a few cents more a gallon for 89 octane after the first piston and liner you get to purchase for your equipment. $300 is typical and can be higher. Alcohol actually leans the mixture and without boring the main jet the mixture just cannot be rich enough to prevent "Lean Burn" and pre-ignition resulting in melted pistons and scored cylinder walls.
An old thread I did a couple of years ago that shows how to test for alcohol in fuel.
http://www.mytractorforum.com/showthread.php?t=146217