Toluene would be safer if you really MUST use an octane booster. Toluene is already in the gas(5-15%) so adding a tad more won't bug much.
MMT will also coat your combustion chamber and plugs. They will turn a very charming shade of pink(think like blush pink). I used a bottle of Gumout Octane Booster once and it coated my plugs in one bottle. Saw the plugs a few days before and 1 week after. Went from perfectly clean to pink.