It can be difficult for an actor or director to decide what to do with Steel Magnolias accents just based upon what's in the script. There's actually quite a bit more background information that makes choices with Steel Magnolias accents a little easier to figure out...
Steel Magnolias - the Play
Steel Magnolias was written by Robert Harling, who is originally from Natchitoches, Louisiana. He uses a fictional setting for the play - Chinquapin, a town in northwest Louisiana. The town is supposedly also located in Chinquapin Parish, which is the Louisiana equivalent of a county. Neither the town nor the parish exist.
Harling was born and raised in Natchitoches (pronounced NACK-uh-dish), and the play is partially based on the events surrounding his sister's death and on the people he knew in his hometown. It's focused on six women, and the entire play takes place in a beauty parlor. The only other character is the male voice of a radio announcer.
Steel Magnolias - the Movie
The film version of Steel Magnolias was adapted by Harling who also appeared as a minister in the film. The cast expanded beyond the six women and the setting far beyond the beauty salon, though they are still the focus of the story. It remains placed in the fictional Chinquapin Parish.
The accents in the Steel Magnolias film, however, don't necessarily directly reflect Harling's hometown. Perhaps the biggest shift from the sounds of Natchitoches is the accent Julia Roberts uses for Shelby. Roberts, originally from Georgia, takes on the stereotypical American Southern dropped-R accent.
The degree to which she takes the accent doesn't reflect the sound of Natchitoches speakers. In fact, it plays very much into the breathy Southern belle stereotype that doesn't tend to exist at all outside of Hollywood films.
Steel Magnolias Accent Recordings
I had the opportunity to coach accents for a production of Steel Magnolias a few years ago, so I traveled to Natchitoches, Louisiana. I had the luck of recording a wonderful example of a Natchitoches local who had a wonderful working-class accent that would be very appropriate for the character of Truvy. This is included in the Hard-R Southern accent download on AccentHelp. I also was able to record her saying quite a few terms from the play.
I also was able to meet up with a colleague at the university in town who took me along to a ladies' book club - GOLD MINE! As she stated before we went: You'll find all of the characters from Steel Magnolias at this book club.
The characters were not literally there, but they were a perfect representation of the archetypes of the play's characters. And they were gloriously open to sharing tales of Natchitoches' history and various events the ladies all had a hand in at one time or another.
These Steel Magnolias accent recordings are available on AccentHelp, and include over 45 minutes of conversation along with comments from an expert on the play and the city. In addition to these recordings in Hard-R Southern, there are additional Natchitoches (and Zwolle) recordings as well.
Funnily enough, even Dolly Parton's Tennessee accent was more accurate for the Steel Magnolias accent than the southern others took on for the film. The twang of Dolly is very much like the Truvy-esque accent included in the Hard-R Southern. My true-life Truvy even commented on Dolly being the best of the Steel Magnolias accents, capturing the character largely due to perfect casting...