If you're going to build models from balsa it's good to learn what makes the material tick.....

http://www.mat.uc.pt/~pedro/ncientif...s/techbal.html
http://skyhooks.ca/balsa.htm
.... and in particular this page with pictures of the grain types http://skyhooks.ca/balsa3.htm#anchor446684

C grain has the advantage of being very stiff in bending and quite a bit stiffer than the other grades at resisting twisting so it's perfect for many uses where you want the wood to stay as flat as possible. A grain is what you would use for forming cross grain curves for rounded fore and aft decks on curved top fuselages.

And balsa also comes in a very wide range of densities. Until you get a feel for the weights it isn't a bad idea to buy a small gram scale and take it with you when you go balsa shopping along with the sheet weight chart. Buy only 4 to 8 lb/cu foot wood for your models unless you need some of the heavier stuff for specific uses.