I could just be bias because on the maps I have played so far I have started in fairly central locations but I find that there is always more to be gained from keeping boarders closed and denying competitors access to new systems than risking opening them. I know some games are built with the assumptions that factions will eventually open their boarders and the AI gets wonky if you never do and I was wondering if that was or was not the case here. On one map in particular while playing as Horatio my Homeworld and first two colonies had influence bubbles wide enough that they blocked passage along the primary north/south star-line in the constellation next to mine, completely cutting the Hissho off from more than 5 systems and preventing every other faction but United Empire access to one or two different Worm Holes pending on their location relative to me.



I know that's a bit of an extreme case but I've had similar experiences to a smaller extent on other maps are Horatio as well and just think its to much of an advantage to ever risk opening my boarders to someone once I have peace.

This is just from my experience and interpretation, but if you don't open your borders to someone you have peace with, you starting getting "Border Conflict" messages, and their opinion of you starts going down. I was sandwiched in-between the Sophon and Horatio, mopping up the Horatio who had declared war on me. Usually I do ally with the Sophon just so I can get some advanced tech from them through diplomacy, but this time the most I had was peace.



I started getting those diplomatic notes, specifically as the colored bubbles around my systems got larger and started impacting with the Sophon's bubbles. I didn't think much of it and let the Sophon drop from Very Close to Suspicious. They broke a few trade agreements, and war followed about ten turns later.



So, maybe if you want a lasting peace, open your borders to someone you plan on allying with? Note, I play slow, 310+ turn games, so it may not matter for faster ones. I just like slowly grinding my enemies into dust.