I was interested in PoppaGator's comments there. When I first read "Waiting for Godot," I commented to an actress,"I can't imagine it being performed with anything other than a soft, Irish accent, because the rhythm of the language just wouldn't sound right any other way." She promptly pointed out that the play was actually written in French and translated! Am I right in believing that the version of Nabokov's "Lolita" which we know, is actually a translation from the Russian by his son? I don't know what the linguistic experts say, but I think that rhythm is as important as sound in a dialect.
Personally, unless I think I can make a good stab at an accent, I bend the words a bit until I think it sounds credible coming from me. In particular blues language and words like "mama" sound quite ridiculous from my lips, so if I am singing something like Tommy Johnson's "Canned Heat Blues", I leave them out altogether.