I looked at a few chapters in Deathly Hallows before trying to comment just because I was curious to pay attention to how JKR writes their dialogue. From what I noticed just in this one book alone, she seems to write Fleur's dialogue phonetically most of the time but not soo much for Viktor. Viktor's phonetic-speak only shows up every once in a while (though granted he is not in a lot of the book). Hagrid kind of falls in the middle of the two. I found that interesting.

Personally, when I first started reading the books (and fanfiction) it helped me keep character voices straight in my head when words were written out phonetically. Because honestly, sometimes I have a hard time getting accents to stick when I'm reading. Of course, people like Viktor and Fleur are mostly locked in at this point so I guess I could go either way.