My materials are chosen for beauty, structural integrity and tonal character. Mostly this is reflected in my choice of tonewoods, but it is also reflected in my choice of tuners, nuts and saddles, etc. While the choice of tonewoods affects the sound, the design and sensibilities of the builder have a much greater impact. With that said, it is important to select high quality wood that will allow the guitar to last for generations.
The soundboard in particular needs to be extremely well quartered to resist the tension of the strings over time. While you may be familiar with the terms AAA, AAAA, Master Grade, etc., these terms are defined by individual suppliers and do not correspond with any universal standard. That is why I work with a small group of select suppliers with high standards that I can trust.
Woods chosen for the back and sides of the guitar have a much subtler affect on sound, therefore aesthetic preferences can play more of a role in their selection. The most important attributes affecting sound are density to stiffness ratios and damping characteristics. Woods with low damping characteristics such as the rosewoods and mahoganies will have a clearer, more ringing tone, while woods with higher damping such as walnut and to some extent koa have a drier, woodier sound. Denser woods such as ebonies and rosewoods will reinforce the low end, while less dense woods such as mahogany will enhance the midrange and high end. Of course different woods have a mix of these attributes and variations within some species of woods can be considerable.