Apparently, Tamaki is ambidextrous (and very capable of using both of his hands).
In Top Caster he was a right hand man. In Nodame, he uses both hands. He chops, uses his chopsticks, writes, and conduct with his right hand while he does other things like holding his bag, shower head on his left hand. It seems his dominant is on the left though (proven by the follow vids).
Games competition for Top Caster
The reversed version of rock paper and scissors had Tamaki confused. He messed up all 3 times. That was too cute. (He used his left hand.) He also used his left for the archery. Shosuke really has great physique. He's so serious and he gives me the impression of having such dry sense of humor, very hard on himself as well.
http://www.youtube.com/watch?v=YYgHP-Jt3O4
Archery competition for Nodame
Tamaki used his dominant left again and almost hit bull's eye. Very nice. (Eita didn't do too well.)
http://www.youtube.com/watch?v=2eRiuF0Hjrk