doh! for some reason i thought this was a free-show, so ill scratch my post.
ive only seen shows at summerstage this year, that were outside (free and some not free). summerstage had people in line way earlier than 5pm door times for Decemberists. That is when doors were. Actual show began at 6:30. I arrived a little before 6pm and it was insanely crowded already.
id guess its prob somewhat similar at mccarren. i have friends that went to the free shows there, but i dont know if that will help you at all.