[Soyweiser]

Going AI:
When you only have one last fort without any commanders/mages/troops in it. Go ahead.

At shrapnel:
Ask the game admin. But generally frowned upon.

At dom3mods:
Never do this. Get a sub/fight it out. (The idea is, that you can still learn a lot form desperate last stand tactics and tricks, and going AI isn't fair for non invading players).

At SA:
Go ahead, you are playing the game to have fun. If it isn't fun, you should stop.

Those are my general conclusions.