I generally construct a ring of bases around the outside, and possibly one in the center. I only place one in the center if I plan to NOT use all of the productions squares at any point.
The crater is very helpful to bases, especially small ones. Placing in a ring around the outside allows three-four square for each base that have +1 mineral.
One of my fav tricks once the crater is mine is to drill an aquifer and more often than not all internal squares make a nice circular river (kind of defies physics) but allows a nice +1 energy in all the squares.