Pressing the home button to exit an application does not close it on the iPhone. It keeps the application running in the background which in turn effect the battery life. Here are the steps the fully close running applications on the iPhone.
shouldnt help much, considering apple doesnt allow (most) applications to run in the background... the very few things that are allowed to run generally dont kill battery much... a few things that are allowd to run in the background are Music and I think some location services (that one might drain some battery) but the majority of applications actually stop processing.
This is the reason why apple did not add a "kill all programs" feature because they have it designed to not really make a difference.
Often not necessary, the whole issue isn't that simple:
BTW, I have over 50 apps sitting in my drawer with no affect on battery-life.