Mirroring Question in Lego Modeling Tutorial
 

In Part 5 when mirroring gets introduced, the tutorial has us deleting faces where the object will be mirrored. I understand that part, but then it also says to delete the side faces up to the smoothing control edges. Wouldn't this make the object subjectively shorter once mirrored?

Alternatively, would it be better to delete just the faces that would be adjacent when mirrored and then seperately delete those control edges?