Combustion temperatures are too high for most cars if you run more than 96 octane. Probably ill advised to go more than 50/50 racing blended with 91. But using 50/50 every now and then shouldn't send it into melt-down mode. Unless the ECU is mapped for the higher octane though, you're not likely to feel a difference on any pump/racing blend beyond that's north of 93 or 94 octane. Most people I've know that used pump/racing blends on occassion also had multiple maps in their aftermarket or piggy-back ECUs (usually mapped the pump/racing blend at 94 octane just to leave some safety margin). Factory maps will still treat a 96 octane pump-racing blend like it's 91.