It all depends when you are going.
Yes, sometimes parks open a bit early...I have experienced the parks open up to 15 minutes early, though not the entire park would be open.
For instance, I got to Universal Studios and the park opened 15 minutes early. Only Shrek and Jimmy Neutron were open before the official opening and various employees stood guard and would not let anyone pass them until the posted time of opening. However, this can be an advantage...get one of the most popular rides out of the way before the park is official open.
I say it is indeed worth it. I have gone on Hulk 2 or 3 times before the park officially opened, which was neat.
Of course, this doesn't matter as much if you have FOTL, but I personally try my best to get there when the park opens...but again, most of the time I don't (sleeping at RPH...too hard to get up...I, who love going to the parks, sleep in until I risk missing the first Ghostbusters Show...then I'm up!!!

).
I say, go at opening at least once...less crowds, and if you want to take great pictures, it surely helps...I sometimes like to NOT have people in my scenery pictures...

But, it's just much more enjoyable with smaller crowds.