Yes, it is totally okay to leave your subwoofer on for as long as you want. This is because your active subwoofer has an automatic power-off feature. Whenever the receiver is dormant, your subwoofer recognizes it and enters an auto-standby mode.  But if you are not going to use your system over a long period of time, and want to turn it off manually, that’s okay. Otherwise, there will be no harm caused. It’s really up to you.