I doubt it. Android does not work that way; activities that aren't being used don't run. They exist in memory, and "killing them" removes them from memory, but that doesn't affect battery life.

Now, if you install every app from the Market and don't use them, then that can be a problem, because some apps do run in the background. But that is a very special case. Things like Maps, Browser, etc., aren't eating your battery life. Things like updating Twitter every 30 seconds are. Telling Twidroid to update your tweets every 15 minutes instead of every 30 seconds will save more battery than all the task killers in the world ever will.