Softer back wood works best with softer top wood; otherwise the soft back will not “drive” the top well for volume and projection.
Harder back woods work well with a range of top woods; a hard back will fully drive a softer top, and will still drive a harder top well.
For maximum volume, headroom, and aggression, choose a hard back and a hard top.
For delicacy and subtlety, choose softer, lighter woods.
Choosing woods from the medium-hard range, such as rosewood and certain types of spruce provides a good compromise of responsiveness versus volume and projection (and is likely the reason this pairing is so popular).