I tend to play on small clusters maps, so I'll capture my 4-6 starting sector's planets and fortify them. After that, I start exploring nearby clusters, grabbing ARS, Flagships, Reducers, and tech vaults. I'll hold onto the occasional system with a military station if it's surrounded by other goodies and high mark planets (The 3x factories help attacking worlds adjacent to the military station). They also help propel your forces deeper into AI territory to get some of the farther-off golems and officer flagships. So, in my case of playing on small clusters, I might have one military station every 3 clusters or so just to give my fleet a local place to repair up if needed. Support fleets are good for this, but as of 1.002 the AI really loves murdering support fleets.