Hang on to the money

I HAVE just had the 150,000km service carried out on my 1997 Mitsubishi TF Magna and the mechanic told me I should consider having the fuel-injection system cleaned at a cost of about $180. Though I did not buy the car new, I ran it on 98 octane almost exclusively from about 30,000km to 110,000km. Since then I use 98 about every fourth fill. Doing mostly city driving, I achieve 10-11 litres/100km. Would I be wasting money cleaning the injection system?