I follow a general rule of thumb: "up-left and down-right". Basically, control signals migrate upward on the left side, through the modulation/timing, then up to the VCOs at the top. VCO signals then go toward the right and downward; in smaller builds, the VCF is likely to be on that same "voice" row but if the space is larger, I tend to put VCFs between the VCOs and the mixing/output area at the lower right. This also should leave the VCFs and some of the VCAs in proximity to the modulation (envelopes, especially) for convenience and to keep the cable pattern somewhat sensible. The idea basically is an adaptation of the flowpath of the ARP 2600, save that modular architecture allows for considerably more flexibility in function placement than the 2600's fixed panel design.