Tires need to be replaced if the threads in the casing get cut, or before the tread wears down to the casing. If the wear on the tread is uneven you also need to check for something out of adjustment in the wheel or brakes.

The tire direction of rotation is supposed to be with the arrow pattern in the tread pointing forward at the top of the tire. The tread on a road tire is really for decorative purposes only, so this isnt important.

You always want to have your best tire on the front, to maintain control in case of a blow-out. Some people put their front tire to the back when the back wears out and put the new tire on the front. Back tires wear faster than the fronts because of the extra weight on them and the driving force.