Frontier is great to j/s on. Any seat in the back is available. It helps if you call their reservation number and list for the flight. Tell them you're j/sing. Both times I'm j/s'd the F/A or Captain gave me free TV service. Thanks!
I've gone to the ticket counter before and they gave me a boarding pass for the wrong flight. The gate agent will give you your boarding pass, so need to go to the ticket counter unless you don't trust TSA through screening.
I jump seated to DEN on NW from MEM a couple weeks ago. There were 3 of us on the flight, all in the back.