I don’t like it when people affect accents when they sing. They should sing with their voice not someone elses. I guess there are exceptions to this, if you are doing a character, but aside from that I think putting on an accent, especially an american one is bad. This is one reason why Kate Nash annoys me – when you hear her speak she is very public school but as soon as she sings she’s a cockney. What’s going on there. Mike Skinner (from The Streets) is another interesting one. He is from the West Midlands but he raps with a London accent, apart from the odd word (like “dance” in “Let’s Push Things Forward”).
Some people argue that singing with an accent is natural and makes it easier to sing. I don’t agree, it’s just habit. This is what Joni Mitchell has to say on the subject:
I find a lot of black affectations irritating. White singers sounding like they come from deep Georgia, you know? It always seems ridiculous to me. It always seemed to me that great singer – now we’re talking about excellence, not popularity – but a great singer would sing closer to his or her own speaking voice. (from here)