I have heard and read what B.diddy said many times, but I have found that not closing unused apps causes more battery drain than closing them out.
For me, it is SOP to always close out, "back out" of apps that I don't want running. and I always hit the Recent Menu and Close All.
Also, when my phone goes to sleep, ie, the screen is locked. The internet is turned OFF totally. no WiFi, no Data
MacroDroid turns internet on/off by monitoring the screen status. All I have to do is press the power button and everything shuts down.