On a drop weight the error comes out to the cosine of the angle from level and the error is always less tension.
So at +/- 10 degrees off of level (which is a pretty big range) you're only introducing 1.5% of error in tension. With a lockout machine with a stretchy string, you're going to be off by way more than this.
As for the drawback, I'm not convinced this is that big of a deal either. If you pull tension and get that tension around the 180 degrees to the previous string, you'll pull all the drawback back out of the string (ie the clamp will be straight again).
I think there's way more loss in string to string tension pulling crosses and the string naturally stretching after pulling tension to worry about the maybe 2% total loss you'll see from drawback and ever so slightly unlevel tension bar.
As long as you're consistent with your technique, your error will be consistent as well... this goes for a $5k machine or a $100 drop weight.