Alien bombing runs start in June, if I recall correctly, so you should schedule your bases to have some torpedo defenses by then, and getting them built should be a priority for new bases as well, though not as important as in your primary bases *). Because the accuracy is quite low, for safety you should build preferably at least three. Being able to upgrade the defenses would be excellent.
*) For example, if your major base subpen gets destroyed when the craft is out on a mission, the craft and its crew are destroyed as well. I had this happen once in an SH/IM run..
So speed up the build-up, in new bases I think I have usually built an outpost and/or a corridor among the first and then expand to the squares next to it. You can scrap the outpost after you have build proper stores and living quarters.