Actually, after some tests, I must admit I'm not a huge fan of the keys because they ad new problems to solve.
If it's a negative key, it's pretty OK wherever it ends up, but when it comes to positive keys, here comes the troubles because if it's on the bottom, then you'll end up with a total overhang and sometimes it's just easier to glue two perfectly flat surfaces and then get rid of the gap by sanding and "epoxing" it, operation that you'll need to do anyway with keys.
I also hate supports, they slow down too much the print so I simply end up by slicing the model in order to get as less overhangs as possible.
I did this way on a puppet without using keys and it ended up quite ok.
It's made of 2 pieces for the body and other 2 pieces for the arms.
Forgive the links flood but it's just to show an example (that maybe it's more on the postproduction side).
Here you can see the line between the top and the bottom
http://instagram.com/p/m5GLTTFdNy/
after I glued/epoxy putty/sanded/primered it:
http://instagram.com/p/m-z2djFdAj/
first copy after I molded it:
http://instagram.com/p/nLq-snldH9/
The overall surface came out smoother than expected.
The only "key" I used is on the arms and it's not actually a key, it's just a boolean of the arm into the sleeves, so it will be easy to attach the arms on the body (once painted) without having to deal with seams
http://instagram.com/p/n0TyW9ldHS/
BTW, this one was printed at 0.06 and it's something like 15cm tall so I had to sand it a lot, since then I'm printing at 0.04, I'm lazy and I just dream of a world with a simple primer pass to get a smooth surface, which is pretty much all you need on a good print at 0.04.
Sorry for the long post, hope it helps.