My plan is to build bases so that I have complete radar coverage over the entire world, which is possible. Leave extra room between them because eventually you'll research better radar that will increase each ones range.
Base 1: One of three places, Middle of the good'ol USA
, middle or Europe, or the middle of Asia, closer to the Pacific than Europe. This base holds my A team and usually ends up being a hybrid that specializes in research. Usually put two interceptors here.
Base 2: One of two places I didn't put my first base, usually on the other side of the Atlantic. This one holds my B team and will be a hybrid base of production and research. Two interceptors here too.
Base 3: The last of the three places mentioned. Now I go with a full production center and maybe in the mid game start working on a C team.
Base 4/5: Middle of Africa, Middle of South America. These bases will also be production oriented for later on building aircraft, with room for large hangers. They will also have a little research (because you can never have enough research) and a lot of storage for materials to build the aircraft with.
Rest of bases are mostly radar and storage facilities that cover the rest of the world, like in Australia, North Pole. I may or may not man them. The game is usually over before I feel like building the last base.
I build UFO storage facilities at each of them. I like to build SAM traps around one or two bases, with SAMs everywhere. They will miss a lot of UFO's, but when one flies through one it's fun to watch it get beat up continuously. It's even sweater when a harvester changes direction as it's just about ready to fly out of the trap only to fly back through it.
Another reason I don't try to build SAM's around every base is because one SAM site can't usually bring down a UFO, so when a UFO attacks a base, it's almost always going to get through. If you consider that if you tried to protect each base, you'd only have 2/3 per site, and that's not enough to shoot down a UFO before it reaches the base, unless you build a lot of defenses along with it too, but that takes up valuable real estate. I'd rather just kill the aliens once they land in a base defense mission.
To be honest, it doesn't matter too much where you place your bases. You'll be intercepting plenty of UFO's if you want to and fighting them on the ground. The more you spread out your bases though, the more chance you'll have to detect them. Sometimes, having complete coverage is a pain because you're always tracking a UFO somewhere and sometimes you just want time to pass for research to finish.