Rooting will not in and of itself effect your battery life .... think of it as getting access to the administrative account on a computer versus a standard user account.... there would be no change in the performance of the machine by doing so.
What you can do however, is uninstall some of the bloatware, install custom kernels or ROMs with tweaks for battery life, and apps like SetCPU. All I have done is install a custom ROM and my battery life seems to be the same if not slightly worse than standard 2.2 OTA.