To build light and strong, wood selection is very important. You need to choose for lightness and strength. The strength I am most concerned with is torsional strength. On today's large pattern planes, the airfoils, especially at the tip is very thin. This allows the wing to twist easier than thicker wings. To help counteract this problem, I only use C-grain balsa. Though some may argue that A or B grain has more tensile strength, that is not a concern as will be seen further on in the construction with the use of carbon fiber. However, C-grain adds a great deal to the stiffness of a wing. It is more fragile to work with, but not to the point of being a problem.
The next concern, weight and density. These do not always go hand in hand. Hold a piece of 1/16 sheet up to the light. You will see shadows of wood fibers. I look for the fibers to go end to end, and not just part way or in a half moon off the sheet. It does not need to be solid dark, just dark fibers mostly in a straight line. A good piece of C-grain will have a mottled or marble appearance. Refer to an old Sig catalogue for more on this. To get good C-grain throughout the whole sheet, I usually only use 3" wide sheets. It is not easy to get a true C-grain in wider sheets than 3". I weigh each sheet, and average 10grams per 1/16x3x36 inch sheet.
To get the wood I desire, I have special ordered wood to a particular specification and after receiving up to 75 sheets, I still inspect all and only choose 20 or 25 sheets and sell the rest, as it is not good enough for me.