If your recevers in the apps, like say facebook which will launch at device boot, and device runs media scanner, and device mounts sd card. Are active, there are more, say end phone call state perhaps, are active. Then killing apps dose not do alot of good.
You must also turn off the desired recevors for auto run.
You can do this using romtoolbox I belive, or an xposed fraimwork module.
You can also run greenify, thats all I do on my note ii and I dont worrt about ram at all. Ever.
I also use a task killer but only for closing an app when there is an issue with code bug. Or browser errors. It is easyer to just kill everything [as I keep things unchecked that I dont want off] then going to force close it in the settings.
Thats about all a task killer is good for now adays.