The official time is 4:30eastern for the ES however many traders use 9:30 opening so it matches up with the pit trading times of the big sp contract. It's a matter of a person styles mostily. Personally I trade gap opening and the only way to do that for my style of trading on the ES to to use the pit times. Gaps it where I make the majority of my money so I trade that way.
It's been discussed several times at length before on ET so a search should get you more opinions.