Some apps, mainly music and gps, run in the background and use power. Especially the gps. Most apps just save their state in memory and don't use power. Sometimes I have 40 or 50 apps in the multitask bar. If I am not using the gps or music apps I close them. The others I leave alone.

If you have a problem and think memory usage is the issue, download and use a memory freeing app. Usually the ios cleans up pretty well on it's own, but podcast downloading directly to phone is one area memory freeing seems to help.
🙂