When building a droid, part of doing your research is getting to know every single piece of the droids used in Star Wars. After a while, all those "thingys" and "do-dads" are known as "Holoprojectors" and "Shoulder Buttons." One of the inconsistencies you must deal with and decide on when building a droid is the placement of the shoulder buttons on the horse shoes. Many builders place the buttons to the front of the droid while others have the horse shoes postioned so that the buttons are near the back. So which one did I choose?
I choose to place the buttons towards the back for a number of reasons. First, I liked the look of it better. Don't know why, and its certainly not scientific, but I seem to favor that positioning. Secondly, and most importantly, I looked to the films for evidence. From what I could find, the majority of droids in the films show the buttons towards the rear. In fact, the only move that shows the buttons in front with any prominence is A New Hope. Even then, we can find multiple examples of buttons back. The best examples of this is the R2 used in the throne room and some of the droids rolling around Mos Eisley.
In every other movie, I found nearly all (I say nearly because there is rarely any certainty in movie props) of the droids including the CG and robotic R2-D2s to have the buttons towards the back.
That being said, 99.9% of the public will neither notice nor care. People who are obsessed with Star Wars won't even give such a detail second notice. But that is the beauty of building. You can choose the style that you like best. Only a builder will notice, and even then there is no right or wrong.