Do some of the Tour de France riders opt not to wear a helmet during different stages?

Question

Or are they now forced to wear it all the time?

Answer 1

UCI requires all cyclist to wear a helmet in all races.
The only exception is when the race has a mountain top finish, they are allowed to remove their helmet, once they start the final climb.

EDIT:
Also the death of 96 Olympic Champion, Fabio Casartelli in the 97 TDF.
He crashed on a mountain descent and suffered and open skull fracture that was fatal, after hitting a stone wall.

Answer 2

Dennis is correct about the UCI regulation. It was adopted in 2003 following several well-publicized deaths, including that of Kazakh rider Andrei Kivilev who died of a head injury without a helmet. The UCI had tried to bring it in earlier but the riders had rejected it as a group. While I miss seeing the guys in the traditional cycling caps, I think everyone recognizes how dangerous the sport is. Saying the riders are forced to wear a helmet is not giving them due respect. Robbie McEwen got hit from behind in a pileup during one of the stages he won last week. He got flipped over his handlebars and was probably glad to have protection on his noggin. Credit Agricole's Saul Raisin is lucky to be alive after the head injury he received racing last year - the helmet made the difference. You should check out his story.

Answer 3

http://www.dummies.com/WileyCDA/DummiesArticle/id-3082.html

Here ya go!