What Ken says is very true - and not only sticks, but sheets too - Very often you will have several 3" x 3/32" x 36" sheets and a few 3" x 3/32" x 24"
The instructions may say something like:

"From a sheet of 3" x 3/32" x 24" balsa, cut 5 pieces 3 1/2" long each"

That will leave you with about a 5 1/2" piece of scrap

But if you accident;y cut it from a 36" sheet, you'll be left with a 17 1/2" piece

Now later, you'll need to cut a 34" piece from the 36" piece and you'll have a 24" piece and a 17 1/2" piece... Time to make a trip to the LHS!

And PS --- If you do it, don't feel bad. Even though I KNOW better, I still do it occaisionally