You might also look at the balse being used. Crazy as it may sound, some pieces are stronger than others. Dried balsa is the vegetable kingdom's version of foam, that's why it is so light, and so soft. So when you are building the sections of the plane that are going to be contributing to its structural strength, use the densest pieces of balsa you have.
How do you tell? Denser wood is heavier and harder. So if you lack the means of telling which piece weighs more for the same size, then use the fingernail test. The balsa that dents easier is usually the less dense, and therefore the weaker piece.
The first thing I do when building a plane is to take all the sticks and seperate all the same size pieces (for example, 1/8" x 1/8") into two piles; one hard and the other soft. The hard pieces I use for what I can see from the plans are going to require the most strength, like spars, and the softer, lighter weaker piece for "non load bearing" members.
Obviously with the precut parts you're pretty much stuck with what they give you, unless you want to go to the hobby shop, choose your own wood and cut the pieces yourself, which is what a good scratch builders do.