Nope, from personal experience which isn't a lot of experience anyway, they don't work. Apps are quite restricted when it comes to what they can do and an app claiming to be able to alter system information like that are simply bluffing. If want to preserve your battery...
- Close any app when not in use
- Reduce the apps using your location
- Stop apps from running/refreshing in the background
- Don't have your brightness up too much so it's really bright - control it
- Using games like Angry Birds use a lot of system resources which drains the battery quick
Some really obvious stuff I know, but an app won't increase your battery life. It might give you some tips like above though.