When it comes to singing, I think you have to work with what you've got (unless you're Tom Waits :p )
Your voice is unique and it's how people will identify you as a singer. If there's one thing I really hate it's British singers with American accents. What's wrong with your own accent? With your own way of phrasing?
And I think it's true of songwriting, too. The more songs you write, the more of your own voice you develop (I'm talking about a writing voice, now.) All the best songwriters (in my opinion) have a very strong writing personality. It's part of the integrity of good writing.
Of course, having your own voice doesn't mean you have to be limited in style. I would say my songs aren't incredibly diverse, but they're definitely not tied into a style. I write a song, and then play it to it's best advantage and sound. A song deserves to be given room to be itself in a performance.
