The thing is, Android manages apps differently. Even though an app is running, it's put into a sleep state if it is not active. So it'll just rest in the memory. I've read that CPU cycles is what consumes more power rather than apps stored in the RAM. Android automatically ends apps with a low priority to free up space. If you use ATK, then it's better if you end the apps manually rather doing Kill All Tasks.
I remember there was an issue with alarms not ringing when the phone is in standby. I faced this issue while using ATK(free version), but the issue never occurred once I stopped using it. Another issue was my EDGE service would not connect cos it would say 'Mobile service is not running' or something like that. This has happened extremely less after I stopped using ATK. So use it wisely.