I fully agree that there are certain songs which cry out for an accent of one kind or another. My sense of it is that as long as my poor imitation of a particular accent adds to, rather than detracts from the feeling of the song, then I go ahead and give it my best shot. Sure, there would always be purists who would tell me to just leave the song to those more linguistically qualified; but I think the majority of audiences would lean more toward getting the flavor of the song rather than being picky about the inflections, etc.