I'm building a byron extra I was given and the instructions clearly state to sheet the wings and tail surfaces with balsa. I have built a few other foam core wing kits and have never done one where sheeting was not required. The foam core is not very strong until you add the sheeting to stiffen it up.

Here's the thread I started with my progress so far....

http://www.rcuniverse.com/forum/m_8368652/tm.htm