>>110905
In the case of Osaka, I'd say it's not presumptuous at all. It's obvious Azuma-sensei intended her dialect to be an important aspect of her character. Many of the strips make a point of highlighting the difference between her style of speech and everyone else's, starting from her first appearance. Her very personality is meant to be a stark contrast with the one stereotypically associated with Osakans, so her Osakan style of speech is important for maintaining the incongruity. If she were translated to speak just like everyone else, all those jokes would fall flat.
Now, whether the localized choice of accent presents the same effect is another matter, to be sure. For the American dub, it helps to know that Houston is stereotypically considered to be full of business-minded Foghorn Leghorn types--broad accents, loud and boastful, minds on money. New Yorkers are stereotyped similarly, thus the decision for the manga.
However, it's certainly possible (even likely) that the equivalent match of distinctive dialect and character traits won't exist in all language markets. In those cases...well, a distinctive accent or dialect is still preferable, I'd say, if only so the jokes about Osaka's unique speech quirks retain some sort of humor.