Warning!

Like Graig I also use a copy of the port data in RAM. But care must be taken if an interrupt routine can change one or more pins on that port too. You should avoid interrupts in between writing to the port and writing the copy to the RAM. Otherwise strange things may happen.
The best way to avoid these problems is to avoid a mix outputs that have to be changed by an IRQ and main program on the same port. IRQ outputs may be mixed with inputs without unexpected problems.