I think the answer is, "it depends" I have never seen a Classic airplane DQ'd for it, nor would I ever expect to.
There were a lot of fully-sheeted Nobler-based designs back in the day. Cannot recall ever seeing a fully sheeted T-bird, but there must have been some. It usually depended on the kind of wood a builder was able to get. With great wood a fully sheeted wing would be about the same weight or less with a better finish. Also more durable, less prone to hanger rash, and with a more stable structure.
There were numerous Classic era airplanes that had fully sheeted wings. Modern builds with sheeted leading edges and cap strips hardly get a second look. Don Bambrick's "Fury" earned 39 (out of 40) appearance points at the 66 NATs with a fully sheeted wing, but the magazine plan showed it as an open bay with notes in the text about sheeting. I think the Olympic VI plan also showed the sheeting both ways - there were probably others.
Having said that, I personally would draw the line at sheeting an I-beam. To me a beamer loses about 99% of its charm if you cover up the ribs! But that is still not enough to get you DQ'd.
Ultimately its all in the eyes of the beer-holder...