The 'normal' (some derivatives can be configured otherwise) '51 port can be grounded any time through any operation, so, as long as there is no strong high to the port pin, no damage can occur. If a strong high is applied and the pin is pulled low by code, it will blow.
I have, to the best of my recollection, never blown a port pin on a circuit that has worked (blown many whilie initially debugging hardware).
The cases of port pins blowing I have seen in designs by others all have been related to ISRs not being coded correctly.

Erik