depends on where you are, when its locked.
Like anywhere, it really depends on the neighborhood, and the amount of passing foot traffic (the more the better).
I dont hesitate locking my bike anywhere during the day for short periods. Long periods I avoid the west and north. Nightime I only lock where theres plenty of foot traffic.
I always lock both wheels. Generally dont worry about the seat. If I'm gonna park it super-heavy, drunken crowds (like sunday), I bring a cable along with the U. Dont even bother detaching the front wheel, run the cable from the front wheel, through the brooks, down to the U. viola.