I always do online check-in. It's easy and convenient, and if you can, why not? Every time we've done it, there is practically no line at the OCI counter (although there's usually not a very long line at the regular check-in either).
On my last trip I did OCI and gave them my ETA of 11am. I arrived earlier than that and was definitely not expecting my room to be ready. I was so pleasantly surprised when the CM told me it was and very obviously showed my surprise. She told me they usually have the rooms ready early if you do OCI - one of the perks. They have the packets ready to go for you, and you still get the "Welcome to Disney" information - they don't just hand you the packet and tell you to figure it out (I've read on other threads that guests are afraid they don't offer information).
I also got both room requests I wanted - top floor and 50's section at Pop.