Yes'm
If you're formatting your script for studios, the thing you need to realize is that the first army of people who read you script will be the assistants. The almight Readers. They are all the same. They are all named Amber or Megan. They are 26. They just graduated from college last Tuesday with an MFA in creative writing and now they want to be screenwriters. Yes, they want to be person who are you wanting to be at the same time they are deciding whether or not you are worth giving their dream job to. Strange, eh? So they read your script with a slight disdain for you in the back of their brain and they read your script at the end of a long workday where they were yelled at for no reason. Your story is one of three they have to read in one single night and then type up coverage for. Every Amber or Megan who reads your work is looking for ANY POSSIBLE REASON to reject it as quickly as possible. It's too long. It's too talky. It's too much description. It's got typos. It doesn't follow the BS2. It doesn't have a three act structure. It's too small. It lacks emotion. It's too confusing. Its dialogue is odd.
As for your specific situation: when you finish your script, give it to a friend who's been to college. Ask them to read it out loud. Listen to them. Does our screenwriting GUIDE them toward speaking in the appropriate dialect? Or are they just fumbling along with no flow?
The reason I recommend the "college" friend is because Amber and Megan went to college. So you need to know that this is the breed you're trying to win over. I would say, blindly, that you can represent dialect maybe once per page... if it's needed. You'd be better off reminding the reader at the top of the scene with something like.
ZEEK
(Southern drawl)
Hey, Pa, is we all right to shoot them coons sitting right there?
Or, if your whole script involves stylized speech, you only need to set it up in the beginning of script as a whole, since the reader will be constantly aware of it. Otherwise, the following style is just too exhausting to read page after page.
ZEEK
Ay, Pawwww, we ah-aight shoot 'dem coon settin' rye dare?