A nice thing to do with bases is mothball them. Build 10 or 15 over a huge planet with a land based space yard that is in a quiet part of your empire and mothball them. Then if the situation changes and the enemy starts harrasing that area you can quickly unmothball them and have them cranking out units or ships to fill the holes in your defenses. Just In Time Inventory, SEIV style.
Geoschmo