Since there is no restriction listed on your ticket I would say go early if you want to.
I've never been to MHP, but it will be crowded. If you wait until late in the day you might be in a situation where the close lots fill up. If it were me, I would go early, if they told me my free parking couldn't be used until later I would point out the language on the tickets, but if they insisted I would pay for parking out of pocket. Even if they don't honor your ticket for early parking, at the worst it will cost a few extra bucks. For the convenience and piece of mind it brings I think it would be worth it.