Do you want to be able to clean the back glass? If not then put them close to it so the fish have more room in front to swim around.
All in all it is your prefence. Main thing is to keep them secure as to not fall down and scratch the glass/plexiglass.
Actually I think unless you have a walk around tank you will not want to clean the back of your tank. As was mentioned structural integrity is your primary concern and then esthetics