Personally, if the home planets are spread out, I lock the teams to avoid AI teaming up. Nothing less enjoyable than having several AI join up at your doorstep together - compared to the fact that they rarely ever help you attack 'in mass' on your mutual enemies.

I often wonder how the AI gains each others trust - do they have virtual missions for each other?
I was playing a map with relatively close starting locations and one central dead astroid where all home planets meet. I captured this point and pretty much cut off the rest of the ai from each other - next thing I know is that the two AI closest to each other have teamed up and attack the astroid together.
If this was mission based, they either would have missions to kill things (and since I blocked them off from the rest of the empire, they would only be able to target each other) or send each other aid. Either way, it made no sense as to why they were miracluously allied with each other and 0% happiness with me.

From that point on, I locked the teams during setup.