Depends. Do you want to let TM finish its backup now or not.
If not, shutting down your Mac now and later restarting it will let TM continue once the Mac runs again.
Cant remember if it will resume asap when you turn it back on (again you will notice the change in the icon and if you click it will have the option to continue) but i know it will backup when time come, i have mine setup to backup every hr.