Raspbery Pi GPIO + 7400 on a keyboard matrix. Source or sink?
I'd say sink where you can. its easier to sink as your MCU might not always be able to drive the current that you may need, and if you have multiple outputs that need to be driven you might not be able to pass enough power through the MCU to drive all the output. had this problem a few times with some lighting boards. 
but if you insure that the max rated input power for an MCU IO is not exceed then generally it will sink without any problems.