That's an interesting question - my gut reaction is to say 'no, only if for charity and I was with them, or as part of organised group, eg church, choir, school' but on the other hand, carol singing is a real tradition - and one of those things children always did as a matter of course. I don't think it's like trick or treat, as there's no threat involved. I would rather have littler children than teenagers on bikes like we got this week who bawl out 'Good King Wenceslas' out of tune and then expect a tenner (ok, I'm exaggerating ) . I really don't look at all those christmas cards with carol singers at a door, huddled round a lantern and think 'Well I hope that money is going to charity!'. But I'm not sure I would be comfortable with it either.
Also, meant to add, I would definitely only let children do it round our road and people we knew. When I was a student we used to do carol singing in the pubs to raise money for charity - now that took a bit of nerve I can tell you!
Not door to door, no. My ds1 sung at the local shops with the rest of his class and they collected for a local hospice. But i don't agree with the door to door bit. We have only had 1 lot of carol singers this year and i can't say i miss them at all.