Don't save a spot unless you want to get lynched by the mob.
If you are close to the front of the line and you'd be holding the spot for an hour or so, then you might be able to get away with telling everyone you have a friend that's coming up to meet you, but I wouldn't.
Usually you can go to the bathroom or go get food with no problem. Just make sure you tell the person behind you where you are going. If you go get food, ask that same person and maybe the person in back of them if they want anything. You don't have to buy their food, but they probably wouldn't expect you to anyway.
At the 360 launch last year I waited with a buddy. He was there an hour before I was and as people got on line behind him, he told them he was holding a spot for me. They were cool with it knowing about it from the spot.
One of the guys behind us went and got McDonald's at 6am. He was really nice and asked if he could pick anything up for anyone. Most people politely declined, but the dude came back with 20 Egg McMuffins anyway. He was a line hero.
Usually there isn't a dickhead mob mentality, but use common sense. No one wants to wait online in the cold, at night, when they could be in their warm bed, only to see some schlep walk in front of them when they can see the finish line.