So, sure, if you have an app that's continually getting and recording GPS coordinates, or playing Lady GaGa songs, by all means kill it. Otherwise, stop overthinking this and just chill.
I just leave everything running, I only ever kill something if the app is having a problem. It doesn't seem to affect my phone in any way.
(*) For people who say I was imagining the battery drain and that 3 hours at 94% is good, I wasn't! I don't use my phone much and have no push or email at all. I just looked at my phone now and it has been off the charger for 22 hours and 59 minutes and is still at 99% charge, and that's with a couple of text messages and calling my voicemail twice; this is the sort of battery life that I'm used to getting. Mt 94% after 3 hours of no use whatsoever was definitely an issue.