Flight Attendents and gate agents do not own the jump seat. It is a pilot group benefit, negotiated by the Unions (ALPA originally), spread to the other carriers by the Union Reps and managed by the Captain! Professional courtesy demands you say Hello to Captain, identify yourself and "ask" for the jumpseat. Even if you have an assigned seat in the back -even at American, as I flew with a Captain that went back and got the JS pilot. Additionally, dress as required by the operation carrier. ALPA has an open webpage showing listin requirements and dress code. And someone said earlier, you may end up in the cockpit, if the back fills up. So, even though it may show on the ALPA website that jeans are ok; you have to be in a uniform/business attire to sit in the cockpit.