iOS Multitasking

Fraser Spiers:
There is one iOS "tip" that I keep hearing and it is wrong. Worse, I keep hearing it from supposedly authoritative sources. I have even heard it from the lips of Apple "Geniuses" in stores.
Here is the advice - and remember it is wrong:
All those apps in the multitasking bar on your iOS device are currently active and slowing it down, filling the device's memory or using up your battery. To maximise performance and battery life, you should kill them all manually.
Wrong. Wrong. Wrong. Wrong. Wrong. Wrong. Wrong. There are caveats to this but anyone dispensing the advice above is clearly uninformed enough that they will certainly not be aware of these subtleties.
What puzzles me most is, in my limited personal experience, this is the only (or at least the only memorable) "tip" that friends and colleagues have gotten from an Apple Store employee. I can't think of another thing that someone showed me on their iPhone, iPad, or Mac and then said "yeah, the guy at the Apple Store told me to do this."

Spiers article via John Gruber.