I believe you would only get the extra usage drain if you never restarted your phone after installing a new launcher. Or if you didn't force stop sense. Or if you switch between launchers frequently ( not sure why you would do that tho).
This was for older android versions and things may get taken care of better now. But if you reboot your phone after setting up a launcher then only the default launcher will run. And shouldn't affect battery life much one way or the other.
HTC EVO 4G LTE
Hboot 1.12.222 S-off
User name autogeek on themikmik