I think it kind of depends on the style of case you have, and whether you care about color matching your panels or not. However, I generally agree with what @bopodoq says. I tend to visualize the signal path as going bottom left > top left > top right > bottom right.
Clocks and sequence / pattern creaters, etc create the signals on the bottom and send them up to be modulated, quantized, and played with. The VCOs are top left and send their lovely cacophonies right and then down towards to filters, fx, and audio manipulators. Finally the bottom right has some utilities and the in/outs.
I also like to consider how much I will be playing a particular module, and how many cables are going to be going in and out of it. I keep the modules that are more manipulatable around the outside of the case, to make them easier to play, while the set and forget ones can sit happily on the inside, buried in the spagehetti. Maybe it's just me, but reaching into the heart of a patch to delicately wiggle small knobs is not the most fun part of the modular experience.
Questions: Do you really have an extra empty row? I'm jealous. Also, any thought as to what type of music/sounds you want to get out of this rig? You have quite a nice collection of modules here; without too much tweaking you could turn this into a pretty beastly instrument, depending on what you're goign for.