There are logical ways to lay things out and then there are ergonomic considerations. Both are important. But diving into them first might help you figure out what works best for you.
The first consideration... ergonomics (using this term loosely). Patch cables have fixed lengths. Do you have enough patch cables and are they long enough to patch between the most distant modules. All modules have depth. Depending on the available depth in your case or even certain areas of your case may dictate as to where a module can actually be placed. The power draw might come into consideration if you're placing all your heavy draw modules together in one case and everything else in another.
Grouping. Grouping like modules together makes them easier to find. You may have all your oscillators grouped together, all your filters together, all your effects together, etc. I also try to put most of my modulators (LFOs, EGs, etc.) together with like with like. I tend to have my modulators flanking both my oscillators and my filters. I also try to group VCAs so they are close to everything.
When it comes to mixers or modules used as mixers/attenuators, I will try to have a general clump of them near the ends of the audio signal path, but I will sprinkle them around the case in areas where I know I would probably use one (such as one module near the oscillators). The same for active mults... having one near your oscillators is ace and you probably want one near your sequencing modules as well.
There's no right way to do. But if you're going to try to reorganize your case, you want to organize things in a way where your most common patching doesn't require several meters of patch cable back and forth all over your case with no slack in your patch cables. You never want to place strain on your jacks.