Two things to keep in mind:
1) Most audiences cannot distinguish between poor dialect work and a poor performance. They just sense that something is ‘wonky’ and irritating and they cease to be properly engaged in the story.
2) No other skill on an actor’s resume (not singing, dancing, bareback riding, or martial arts) is so intimately entwined with an actor’s process as is dialect work. Any dialect you use for a performance will always be inextricably linked to every action you play, every intention you pursue. If you want the freedom to do your best acting work, you must have the target dialect ready to integrate* at a project’s first read through.
* You’ll know you are ready to integrate a dialect when you’ve mastered it to the point of being able to extemporize while remaining accurate and consistent.
Pamela Vanderway's Dialect 411 