Along the bottom is the best in my experience, you get less arm strain that way, and you don't obscure the screens when reaching over. I've seen a few devices with shift buttons at the top, and it's not very ergonomic or practical.
For right handers, the bottom left corner is best for shift. That's where it is on all the maschine controllers, and console 1. BCR2000 and push has these on the right, which I find annoying.
So buttons in the lower corners or along the bottom are the most ergonomic in my opinion, at least for modifiers.
On the maschines, the most used buttons are closer to the bottom, with less used ones towards the top. This keeps reaching to a minimum, especially important on the mk3 so you don't block the screens.
On console one, the shift is at the bottom left, within easy reach.
