No they check reservations before you get to the entrance.
You can show up whenever you want to. But it's their decision whether to let you in early. I wouldn't make it 3 hours before your reservation or anything, but I was about 30 mins early and they just waived us on thru when they checked our reservation out. It wasn't very busy at that time though. If it's packed, the situation may differ. There's a site you can go to that shows you on a scale from ghost town to super packed for each day of the month so you can get a better idea of how many ppl will be at the park that day. Just Google isitpacked.com
Reservations are checked at the park entrance not the parking lot
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