Depends on how bad the bend is. If you are really tourqing the board you could damage things like solder joints or traces on the mobo's PCB over time.
Are you saying the mobo is mounted into the case slightly bent? Or you just bent the mobo slightly when plugging in the power cables? If you bent the board a bit plugging in a cable it should be fine, however I would not bend the mobo and leave it like that in order to mount it into the case.