Exception to the rule for Port 0: if you enter Powerdown, P0 might float if not pulled up. this could cause currents in CMOS logic chips.

Pull ups might be required, if you drive for exp a ULN2803 which incorporates pull downs. The port pins do only for a short time have a strong pull up to charge for exp a bus load. Most standard ports garantee 1.6mA capibility to pull down to 0.5V, so I use 3.3kOhm or 4k7 pull ups.

Werner.