If your actor resume includes a section that looks pretty much like this…
DIALECTS – English (RP), Cockney, German, French, American Southern, New York
…any savvy casting director will suspect that you have listed the dialects you were introduced to during an actor training program, and are most likely proficient in none of them.
(They also tend to suspect that you are over-estimating your other abilities…)
Pamela Vanderway's Dialect 411 