The major consideration for me is usually diplomatic issues. For this reason I usually just open borders as soon as I can to build the "Open borders" mod to go along with the "Many years of peace" one. As the Ai is often capable enough in Immortal+ to beat you to Writing, that's just as they get it, more or less.
Rules about opening borders can be petty fluid based on the game and players. There aren't a lot of hard dos or don'ts
One that comes up a lot though is not trading with a hate-magnet if you can. Example: Gandhi in a field of warmongers or Brennus in his own religion starting wars in a mostly one-religion field. The headaches and negative diplo with demands concerning the hated AI are worth a lot less than just giving up trade routes with that one guy. And by staying as hands-off as you can, there's less risk of the others favoring negative attention on you.
Kinda going along with that, I don't like to open borders with a hate-magnet who is a good trade partner, because if forced to cave to the pressure and end relations from a demand gives you a long practical penalty of not being able to deal with them at all for a good while (Refuses to talk, sometimes to the end of game!). So it can be better to just not open borders/trade resources, but still use them for gold and tech trades. You can kinda monitor this one and just manually end the border agreement when you see the hate-magnet start to pick up Worst Enemy statuses, as you won't lose them as trade partner or get demands if you do it with proper timing.
AI has many scouts and he finds me first but I don't for a long time (because of difficulty level). So AI benefits from trade routes, I don't.
On higher difficulties I would argue this doesn't even matter, especially Deity where they are going to press in on you anyway if they have a land path. Being able to check out the AI's land and start foreign trade are nice to look forward to, but most important you will begin to get positive diplo and actually do
something while playing the opening game. Also, they only benefit from trade routes with you if they actually have a route to you and you haven't seen their cities (such as they have Sailing or Wheel and you don't, and have no idea where they are); if you have a connection to a city of theirs that you've seen and it's hooked up to their network, you'll get mutual trade. Still, this small bit is nothing compared to their other advantages anyway.
* 2 AIs are fights each other and ask me to join their sides. I get -1 penalties for each. As I understand they can't demand if we aren't in trade relations... Am I right? Is it beneficial?
Most often I just open with both anyway. If they are actually at war you are 100% safe anyway, and they will almost always hate each other more than anyone else due to ongoing war negative mods...the thing you do have to watch out for is for example a warmonger like Shaka who declares on anybody unless perfectly Friendly and he is starting all the fights without being DoWed on himself -- he won't accumulate lasting hate for anybody he attacks it goes poof when the war is over, so he's just a loose cannon at that point. It's also possible through tricks like city gifting, giving into resource demands, gifting resources and swapping religions (all of which can be done before tech trading) to hurriedly curry favor with one side or another if it looks like you might be next.
Which leads into picking who to friend and who to leave by the wayside again. Shaka on your border is an example of something you likely want to prioritize good relations with; neighbors are considerably more likely to attack you than anyone else, after all. In a game with Shaka and Gandhi on your borders, and Shaka hates Gandhi because he didn't convert him to make him Friendly, you pick Shaka and screw Gandhi, because Gandhi is WAY less of a threat. Or side with who looks to be the bigger threat and not the losing side, especially once vassaling starts happening. You don;t necessarily have to join wars but when the winner just enslaves the loser and moves on, it can pay off to suck up to the winner as you carefully monitor the situation.
* I remember mentioning that Open Borders adds +1 to relations each 25T (and each gifted resource +1 for 50T):
https://forums.civfanatics.com/thre...tive-truth-about-diplomatic-modifiers.655216/ What is correct formula?
It should be +1 every 25 turns up to +2 after 50 turns of Open Borders. AIs can DoW as soon as they have access to 2 units better than archers, which can be quite early on Immortal and Deity (depending on the Civ and map, they start coming by around T70/T50 on those diffs or even earlier), and wars cancel all border agreements, so I find it best to usually start building diplo ASAP.
For resources it's a total of 50 resource trading turns (not just 50turns flat of ANY resource trading) per modifier, and if you trade several at once it raises faster,so giving an AI 2,3 or 4 spare resources is a quick way to build the modifier. Once you get the mod you only have to trade one resource (even in exchange for another, doesn't have to be freely given) to keep the bonus. A nice way to get some additional +diplo is just gift many spare resources when you can afford to, then cancel the deals and only make one trade on the same turn to keep it.
And if you trade enough resources freely, you can even get "Our trade relations are fair and forthright" this way. For fair trade it's important to note that AIs value resources differently: Strategic Iron or Horse are highly desired, luxuries like Gold or Gems less so, Health resources the least. In my experience even the particular AI can have preferences beyond that too (Caesar more highly values luxuries and strategics, for example, refusing to trade you 1:1 on these while other AIs will often trade something like Fish for Gold).
