STM32F303K8T6 custom board
With SWD or JTAG you don't need to power the device for it to flash. But you do have to power the device somehow while flashing. I use JTAG and SWD and built a board with a jumper to either power it from the ST-link or use the power on the board (I think you might be able to buy these). 

Its probably better to not use both to power because the regulators on the board and the flash tool are not going to be exactly the same voltage and you could get power flowing from one to the other. So put an option on the board or flash tool to select use.