chime start imac

All you can do is turn down the volume prior to shutting down the computer, however IMHO it's best to leave the machien running 24/7. There are 3rd party utilities that will mute the sound HOWEVER it is extremely unwise to use them because the Start-Up tone is there for a reason and at some point you may need it.

Yes, many successful users do just that and let the computer or display sleep when not being used. You can set up sleep preferences in System Prefereces - Energy Saver. I only put my display to sleep and have done this with numerous Mac for years with no ill effects.