I don't leave my bike unlocked anywhere in philly at any time...not even for a few seconds. I don't worry about seat theft so much because my seat isn't quick release. I just use two U-locks (one big U for the rear wheel and frame, and one small U to lock the front wheel to the frame). For quick stops I'll just lock the back/frame, but I use both locks for any kind of extended stop. With that setup don't really worry about theft.