EMH are usually accurate adn don't change a lot in my experience.
Park hours are an entirely different matter.
I love Touring plans, but don't necessarily believe their crowd calendars or their expected hours.
Disney can and does change hours even up to a few days before.
I have read 2 schools of thought: 1 to go to a park on EMH days and get there early and another to go the day after EMH as its often less crowded. Not sure I've ever seen any realy data to back up either concept.