Android, by nature, tries to keep RAM full but cpu inactive when the device is idle so when you exit out of an app it may not "close" perse in that it stays in RAM but it's cpu usage drops to none. By killing apps that are using no cpu it can actually have a detrimental effect of using more battery when the cpu becomes active to fill the RAM back up. Task Killers can also add poor performance into the mix by closing things that shouldn't be closed.
Unlike ios, Android does a good job of dumping RAM as needed from idle apps and processes to free up RAM to active ones.
If you're concerned about battery life is why you close things, Wake Lock has a bigger battery drain than idle apps because an app that keeps Wake Lock active keeps the cpu from going into deep sleep. An app called Gsam battery monitor, free on the Play Store, is an excellent one to keep an eye on your cpu usage, Wake Locks, etc.
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