[Edi]

The answer depends on your objective for the game, because Dominions is by its nature a competitive game where there can ultimately be only one. If you want to win, you need to use all the effective means at your disposal, at which point ganging up on others becomes a requirement, as explained to you in detail.

If you don't care about winning, you can skip that and play any way you like and see how far you get. For this reason, single player is better for sandbox games where you can try whatever strategies you like. Of course, the AI will never be as rewarding an opponent as a human player.