Oh yes, I can remember an excruciating rendition of "Wor Geordie's lost 'is penker" by an RP speaker. Bad idea.

Some Americanisation of accent is normal enough in the UK, as it's so prevalent in pop music, and we're very familiar with US accents in films etc. Whether this should be attempted for an American audience is another matter, but most UK listeners will scarcely notice it in an appropriate song. What does get ridiculous is if a UK singer is unable to sing anything without affecting an American accent, as often happens in pop music now.

It's a matter of what you're comfortable with. If you're aware of "putting on" an accent, it's got to be really good if it's not going to sound silly. If an accent comes easily and naturally, there are probably cultural reasons for this, and it may sound OK.

Marje