DE2 board. Flash and sram with NIOS

Actually in your case there's no need of a tristate bus, since you have a single device and data pins don't need to be tristated. This is mandatory when you have multiple devices using the same data lines.

Connecting to sopc tristate bus makes sense because you easily have all connection signals ready without requiring any glue logic or a specific sopc module. The best implementation would be building a custom async memory interface sopc module without the unnecessary tristate logic.