Not going to bother reading all the previous posts. You may have a seat in the cockpit via the gate agent-shes not the final authority-what if the seat/seats are MEL'd, there's low O2 available and a third person can't sit there >>you are there at the Captains whim and permission. Have a beef-dont do it there-get flight number and time and pass it to your ALPA/Union rep.
I ALWAYS go up, I ALWAYS ASK and let them know if a seat was assigned. Be a grown up-ASK.