Hi,
You should add double DFF with all asynchronous signal inputs where you need high reliability.
Thus I'd add synchronizers on data lines.
I personally try to avoid asynchronous signals... this is one reason why I try to run a whole PCB from one single clock source.
The higher the data rate (signal frequency) the more likely are meatastability problems.
In detail it depends on the signals, related signals (data_clock), signal frequency, system clock frequency, reliability...
******
I wonder where you get an "asynchronous data bus" usually there are related control signals.
If there are any, then you may avoid the synchronizers on the datalines.
In most cases the synchronizers won't hurt...
Klaus