In theory you shouldn't need to close apps at all, most go completely to sleep and they shut down totally if memory gets low........
This whole "should I close my apps or not" confusion/debate boils down to this:
1. As posted above by Solubleapps: you shouldn't need to close apps at all . .....I agree with this
2. As reported by numerous users (including myself): some apps continue to run - even when switched to background/closed to "multitask dock" and seems to be continuously running and not in suspended mode (usage and standby times are identical)
The problem is this: When "2" happens, there is no way to tell which app(s) is causing the issue of continuously running. When you double click your home button and see 20+ apps in that dock, how can you tell which app(s) is causing the issue? You cant. . . In this case, its best to just "close" all running apps.
Although, it's USUALLY not necessary to regularly "close" all background apps, I think it is a good practice to do so . . periodically when issues arise