I do the online checkin, but I think the only benefit is if you are arriving earlier than the standard check in time. Filling the arrival time box with an early morning time possibly gets them to assign a ready sport earlier. I normally arrive early/mid morning and indicate a 0700 or 0800 arrival time. I have never received a text with a site assignment until after I was there and worked with the check in CM to see what was available where I wanted. One of the advantages with arriving early is you normally have a bigger selection of sites that were empty the night before. Once they start texting site assignments for the day, those sites are unavailable, even if they are physically unoccupied when you arrive. The only exception to the arrive early/bigger choice is if the Fort was full the night before. This happened to us in January when we arrived on marathon Sunday. The Fort was packed at 0800 when we arrived and there was nearly nothing available. Later in the morning, 15 minutes after check out, half the Fort was empty.
j