How much of an effect does it have on the phones battery life to keep applications such as the contacts application open. I ask because that app sometimes takes long to open the first time depending on how many contacts you have I guess. I'm running android 4.1.2 on an galaxy s3 if that makes any difference. But I'm wondering whats the best practice from an user as well as developer perspective with regard to keeping apps open as a posed to the old school conservative thinking of closing everything when it's not needed.
In Linux/Android it is not recommended to close apps or use task killers. Unlike a Windows computer/phone, Linux/Android can keep applications "running" in the background which doesn't use any CPU or networking (unless it's a media player). It has actually been proven that closing apps causes more battery drain, because instead of Android being able to recall the app that was in memory it has to relaunch it, which requires more memory use as well as CPU.
You are better to leave apps running.