Better to break your word repeatedly early and use it to get off to a good start I think. Aside from an alliance with the Seleucids against Egypt I really don't care for allies as anything but a temporary expediency. I ally with macedon to make the campaign against teh greek easier. i ally with both Greece and Carthage just to empty their vaults and stab them in teh back. After that only the Selucids matter and you can get an alliance with them before you stab the Macedonians in the back.