This is going to be siliconed onto the back, right? Not be the bottom? Sorry, dumb question....but just want to be 100% sure.
I definitely say go with no ledges...they don't look quite right to me, and I think they would make cleaning more difficult. Plus, due to gravity they may eventually fall off, leaving a bare spot in the back of the tank. Unless you are using them for a purpose, and they will be hidden by plants or something else, I say no ledges.. They won't be strong enough to hold anything up like rocks (even light ones) of course.
Personally, I would put the small tiles at the bottom (where substrate will most likely hide them) and use the two big tiles edge to edge for the more visible part of the background. The two big tiles broken up by a row of small tiles looks kind of strange.