I rarely 'do accents' as such, but having lived in various parts of the UK for long spells, I do sing in something-like-the-variation-of-my-accent-as-it-was-when-I-lived-there if it is appropriate. As I've never lived in Scotland, I tend to have a Border accent for those. I agree with Bill D that, for me, the Twa Corbies works much better in a Scots-like accent than an attempt to translate it into standard English.