I don't think the Dominion has any such need to receive or send any formal declaration of war. As Sisko said to Senator Vreenak (IT'S A FAAAAKE!), "the Founders see it as their sacred duty to impose order on the galaxy, their
order." This effectively makes them at war with any species or government that does not capitulate to their rule. Even if the Federation didn't see it as justified for them to have destroyed the Bajoran colonies, I'm sure the Dominion did.