I'm not sure separating hives necessarily cuts down on robbing. Beeks all over the world keep hives in close proximinity. There are advantages to keeping the hives close together. One is if a hive is weaker and failing, an easy way to bolster the numbers is to trade places with a strong hive next to it. The returning field bees from the strong hive won't know the difference and move in to the other hive. My biggest reason to keep hives next to each other is it is easier to remove supers, etc. and set them on top of another hive than to bend down to put them on the ground. It's easier on the back and cuts down on bees crawiling up your pant legs! You can put them as close together as you like. I wouldn't recommend it, but I know of one beek with space issues that does his hives as "double-deckers". In nature bees will often set up in close proximity of other hives. I did a cut out once where the lady called me back and said the bees were back. I told her, "no way" as I could see the bees from my window. I went back to find another hive in the same wall not 2' away from the other hive. Placing them facing a fence or hedge row will take their flight path up away from people if that is an issue. The only time I can think of that requires a wide separation is when doing a split or hiving a swarm from your own hive. I like to keep my hives all together in view of my bedroom window. I find a "bee yard" aesthetically pleasing and having them in sight I can spot potential problems early. Finally, keeping them out of the shade cuts down on SHB's.