IMHO, it's kinda silly to use Japanese phrases in an English fanfic.  I 
mean, the characters are talking in Japanese all the time, right?  So 
the entire story is "translated".  Why would you translate the complex 
words, but not simple phrases like "Huh?"?  Also, it reminds me too 
much of American comics, where every foreigner drops a foreign phrase 
every six words to continually remind the reader that he's from another 
country.

Actually, they are not speaking Japanes eall the time. They throw in
English (and other) words right left and center all the time.

I've been guilty of using the occasional Japanese phrase myself, but 
I've limited myself to "tag lines" like "Darling no baka" that just 
don't sound right translated.

Bad boy! Tut, tut, tut...

Kun-chan...