Yes. Always. It makes so much more sense from a usability point of view - not even from a spam point of view. Just general usability.
If they do not provide a valid email, it will ultimately only cause confusion when they are expecting an email, e.g. they've Watched a thread or forum or when they need an email via the Lost Password process.
They might miss out on important communications from fellow members (Conversation emails) or admins.