Hey. I should share my experience with you. I currently own a SGSIII with stock JB 4.1.2 (only different from full stock is it is rooted)
From time to time i like to see whether Tasker(more directly, plugins/profiles(monitors) taht tasker enables) is wasting more or less battery. I started using AutoVoice to see if was using some battery, using a profle taht checks when power connected, when SMS are received etc etc etc. My recent one is Cell Near State to check and change some profiles around.
With some Tasks turning off the profiles when I dont need to and such, I can reach for about 3h~ screen time on 50% battery. Which means, it "should" reach the 6h screen time. Which is pretty much what I reached before without having Tasker even running. Of course Tasker will use battery when you start doing complex stuff, as GPS, Wifi etc, but from my own experience I would say using Tasker to its mediocre extent(i'm not a programmer, but i do play with tasker alot, even have whole week of Gym exercices planned, and each day it sends it to me by notification what are the exercices for taht day, based on the location - activates when I get near the Gym) and I can roughly feel a 5-10% more battery drain, and yet, that increase might be my own placebo effect! For example today I only have 2h18m screen time and im already at 61%. I might get 2h:30 screen time at 50%, who knows :D
But ye, no need to worry with Tasker if it uses moer batteyr or not, just think, if you had to do that manually it would take more time and creen on time(meaning more battery) to accomplish it. And hell, if you do remmeber to do taht X or Y stuff, u don't need Tasker :P
Hope taht helps in some way feeling u more secured about this great app. Aw another thing, if you got some profile stuck in a loop, that ye, will drain your battery dry :)
P.S: Use Better Battery Stats app from xda forums. You can check everything it does on ur battery. And as listed, Tasker its the lowest of the concerns :)