We've explained before that task killers rarely improve performance on Android phones, and the same is equally true of iOS devices. Blogger and education consultant Fraser Speirs offers a well-explained summary of why shrinking the number of apps on your multitasking bar won't help extend iPhone battery life: because it's a list of apps you've run recently, not apps that are running right now.
Speirs offers a detailed explanation that's worth reading, but the gist is this: the apps you see in the iOS taskbar when you double-tap the home key represent recently used apps, not actively running apps. Unless the app falls into one of a small number of categories where background activity is required (playing music or checking your location, for instance), it will usually stop running in the background altogether within five seconds. Apple itself notes that this means battery life isn't impacted much, but it's good to have the detailed explanation if a friend tries to convince you otherwise.