One thing that hasn't been covered: Pressure changes with temperature. Most people pump up their tires at home but then as they ride and the tires warm up (or cool down during the winter), the pressure inside changes. During the summer I use tire warmers and set my pressure hot. That way I always get the exact pressure I want (everyone knows it is extremely important) with the added bonus that I can start hammering right away rather than having to zig zag down the MUP to get my tires warmed up before the real action begins. During the winter I just pump up (or down) on every stoplight just to be safe.