Apps running in the background use little to no resources, therefore almost no drain on the battery. The operating system is designed to keep certain tasks in the background so they can be opened quickly and easily. It has been stated by many (including me) that task killers should not be used on Froyo and above. They're a thing of the past now. By killing a task that's supposed to be in the background as determined by the system, you're using more battery because the system has to start the task back up. Look at your battery statistics (menu>settings>about phone) and you'll see what's draining you battery.
The best thing to do is restart the device maybe once a week (just like you would a pc) or when it seems to be acting funny. These are mini-computers, after all. To retart, hold the power button until the menu pops up and choose restart.