No battery drain unless it's some kind of service (which is usually (always?)) in the notification bar. When you press the home button, the app will be put into RAM and classified as cache, so it is quicker to open up next time. Note that filling RAM uses no extra battery, RAM is powered the same regardless. If you are using an app that requires more RAM than is available that is classified 'free', it will start removing some of the apps android has put into ram as cache and fill it up with necessary data from the foreground app.
Task managers are useful, however not any more useful than Androids built in managing (system settings --> apps). There is never a need to close any app forceably UNLESS it is not working as it should, whether it be frozen, excessively slow or not loading in some way or another, etc. All other circumstances are taken care of for you and memory is used as efficiently as possible.
Device: Nexus S i9023