Either is fine.

Nakano-ku means Nakano.  Each prefecture has 
districts, and Ku is what that means. Nakano is 
one of the 23 districts that is followed by "KU", 
all other districts are followed by "SHI".  Tokyo 
is different from other prefectures. Its one of 
the two, that uses "TO" instead of "KEN".

Tokyo-To Nakano-ku is the way you would write the 
adderss if you're sending within Japan.

Tokyo Nakano would probably be the way we would 
write it if you're sending to Japan.

RX