No, you are not stressing the PSU.
The nominal voltage for mains power in Europe is actually 220V, not 240. That being said, anything between 210 and 240 is considered normal for supply voltage.
As for the PSU, it is designed to work anywhere, including North America where supply voltage can be as low as 110 volts (60Hz).
The reality is that a standard PSU in a normal computer is converting input power to 12V DC. It's not the input supply voltage that stresses a PSU; it's the load factor of the components inside the computer that stress it. That's why you have to size the PSU appropriately for the computer itself. Your location in the world and the quality of mains power is rarely a concern, except in countries where the power grid is unreliably flaky.