I put mine on the left so that I can use the entire vertical length of the screen for internet and apps.
I always used to keep my Dock on the left for additional vertical space, but the display on the 27" iMac is so large that the left edge seems to be very far. So far I've left the Dock in the bottom due to this, but I might later switch back to left
I put mine on the left so that I can use the entire vertical length of the screen for internet and apps.