You should do online check in if:
-you have no room requests
-you are arriving on a Saturday or Sunday between 10am-2p
-you are arriving during a busy time of year (Thanksgiving, Christmas, Spring Break, etc)
Online check-in can cut down on your wait time and more often than not, they will try to have your room ready for when you arrive. Because of this, they often disregard requests in favor of getting you into a room that's ready. If you ask to change your room at online check-in, your packet has to be re-done and room assignments need to be switched around. Took us about 25 minutes when we did this so I will not do online check-in again.
Well, on the flip side of this, I *didn't* do online checkin for our trip in 2011, and checkin for us at SSR was a *disaster*. The day we checked in, Disney was integrating the new Hawaiian offerings and the computer/reservations systems (apparently worldwide) were totally hosed up. The lines at check-in to SSR were five-plus deep in four different lines and people were sitting and waiting throughout the lobby. We had made special plans to drive into Orlando early and "steal" and extra half-day at DTD and catch an evening in one of the parks...but...
When we checked in, we had been given a "preliminary" room number that we were told to call back and confirm in about 30 mins (the clerk gave us a ph# to call to verify). We did; the person we spoke with had no clue what we were talking about. Then they told us they didn't have the room type we booked (2-BR villa, so they were moving us
down to a 1BR studio. I explained to them that wasn't acceptable). Then they said our reservation was botched up in the computers, and had to rebuild it from scratch as I stood there.
About two hours later, they did finally assign a room..so we get our keycards, drive over, luggage in tow...we open the door, and the room was OCCUPIED!! Suitcases, food, and souvenir bags, the works.
I wasn't happy.
So we drive back to the carriage house, explain the situation...wait a half-hour, and they assigned us a different room...we drove over to it, and this time, the KEYS didn't work...so we drove back to the Carriage House, and after another half-hour, they cut us new keys, and we're finally in. It took us FOUR-PLUS hours to get into our room, and it wasted our first afternoon at Disney. We were so tired, frustrated, and hungry that all we ended up doing was drive ourselves over to Earl of Sandwich to grab something to eat, and called it a night. All that pivoted around the computers being screwed up (among other things).
Sooo, with that sad story in hand, this time, even if its nothing more than checking an electronic box in a database somewhere, I'm going to try online check-in this time around