When the P0 port is used as part of an expansion bus, i.e. the peripheral (8255) is accessed using movx instructions, there is no need to add pull-up resistors, since in this case P0 operates as a totem-pole output.

However, if your program uses P0 as an I/O port (for example mov p0,#25) you will or will not need pull-up resistors depending on the characteristics of the specific external circuit you connect. In this mode of operation, P0 outputs are configured as open-drain. P0 inputs are undefined, if unconnected.