So I'm out over Lake Powell on Sunday cruising at 10,500. We were two hours into a flight and we weren't sure what this front going to do with the conditions enroute. I had planned a flight plan when leaving CO to fly into Vegas. When we get towards Page, AZ I decide given the looks of things I was going to file into Vegas. So I call up Flight Watch but I must have been too low. So I call up Cedar City Radio and asked for briefing and told him I wanted to file. He came back and told me that he would, but I should have filed on the ground before I left. Well at this point, as I said, we were two hours into the flight just into Los Angeles Center's airspace. My understanding is that if I had filed out of Grand Junction with Denver FSS they would have dropped my plan if I didn't pick it up. I could have planned the time to pick it up but didn't. So the question is, was the Cedar City Radio briefer out of line? Can't any body file enroute if they want? If I fie with Denver FSS for a given time but file outside of Denver's airspace will they pass it off to Los Angeles?