I'm also in Hawaii. By all means, do the United website check-in. The counter won't open until 5 a.m. but by then, there'll be a long line, even for just luggage people. And United has cut down on the number of employees at HNL so there will most likely be only one person servicing all of the automated machines. I'd get there by 4:30 a.m.
If you're planning a future trip to WDW, I'd really recommend Continental over United any day. Continental still serves free meals on the Honolulu leg, they have video screens on the chair in front of you with movie choices and video games, they have better flight times (7 p.m. departure versus 11 p.m. departure), and they go through Houston (instead of LAX or SFO, which gives you longer sleep time on your redeye leg - 8 versus 5 hours). Oh, and the Continental flight attendants are not as surly. As you've probably seen, the United Hawaii crews are VERY senior - customer service is no longer their concern. We go to WDW about twice a year and these little things make a BIG difference.