So will you stay at home if your next departure is one where all of the available power is needed to do a "legal" takeoff?
The point is, those legal requirements already have some safety margin built into them. Also, the settings for most reduced thrust takeoffs are calculated using an assumed temperature. So for example, if the thrust selected is for an assumed temperature of 40 C and the actual temperature is 15 C, the calculation assumes the engines will produce the amount of thrust that can be made on a 40 C day. In reality, they will produce more thrust because the actual temperature is cooler - adding even more buffer to the safety margin.