Probably doesn't matter. Technically, the United States hasn't been at war since 1945. However, both Koreas have been in an ongoing state of war since 1950. Despite all of this, the U.S. has been involved in numerous military actions (Vietnam, Iraq etc.), while relations between the Koreas have been relatively uneventful most of the time (a cease-fire came into effect in 1953).
If formal declarations of war are still as irrelevant in the 24th century as they were since WWII, then no declarations of war were ever exchanged between the Federation and the Dominion.