Hmm...but i guess it would be treaty specific too. I mean if we had states that had technology-sharing treaties or right-of-passage treaties (in general anything that affected security), these would obviously have to be revised or annulled since the new larger entity has a different political orientation. (imagine an erstwhile independent entity that had a right-of-passage treaty with the romulans, and then subsequently joined the Federation...obviously such a treaty would become meaningless in the new order)
I think the UFP & the Romulans continued the treaty establishing the Neutral Zone because it was in both their interests to do so.
The Treaty of Algeron (is it possible to get the text of this treaty?) and the use of the cloaking device... it is in the interests of the UFP to have that treaty annulled and so they could claim breach of treaty by the RSE since they've joined the Pact. To further complicate matters the Romulans are no longer a single entity: RSE and IRS...another point in the favor of the UFP.
Since there really is no galactic law or law enforcer, its only political leverage ( tolerance for escalation, diplomatic wrangling) that would probably determine the outcome of treaty disputes in any case. If the parties really wanted to go to war, they wouldn't need a reason (such as breach of treaty)...or could cook one up willy-nilly.