The power settings in the global resources are used to decide the IMO trimming values to be retreived from flash. The IMO of PSoC1 is inaccurate during power up. In the boot.asm file, it is trimmed using the factory loaded trim values in flash to achieve the accuracy specified in the datasheet. These values are chosen as per the power settings in the global resources.
Also, the CPU cannot work at a speed greater than 12MHz (i.e SysClk/2) if the device voltage is below 4.75V. If you select 5V/24MHz and your actual voltage is lesser than 4.75V, you may encounter an unexpected behaviour.
So, the PSoC can work over the entire voltage range of ~3.0V to 5.25V. However, it is necessary to look at the CPU frequency in your project. Setting the Supply Voltage value to the correct value enables the calibration of the CPU speed correctly.