Build Quality is worker dependent...and that likely does not change week to week...but rather on a shift basis...ie 1st shift, 2nd shift, etc. The thing that matters the most I would bet is the components that get used...some weeks may have a bad batch of LCDs resulting in a warmer screen and uneven coloring...or the other components may not fit together as well. That COULD change on a weekly basis.
In the end it's all a guessing game and there are likely good ones in every batch just like there are likely extremely bad ones in every batch. I really wouldn't pay much attention to build week...just get one that works for you.