Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as high-impedance inputs.

Port 0 may also be configured to be the multiplexed low-order address/data bus during accesses to external program and data memory. In this mode P0 has internal pullups.

Port 0 also receives the code bytes during Flash program-ming, and outputs the code bytes during program
verification. External pullups are required during program verification.

The above is from the data sheet of the AT89C51. You can clearly see that you need pull-up resistors in all cases except when using port 0 as part of multiplexed address/data bus.