I know at MSP, they have a special line for checking bags for those who did online check-in, it's a much shorter line than the regular check-in. I flew them last week and could have kicked myself, I forgot to do it before I left the office for my return home flight. So I had to stand in the longer line.
If you don't have checked bags, you can go straight through security, you don't need to check-in again.