Anonymous User - shapper

Hello,

On my web site I have a property, Visitor, which is available for
Anonymous users:

[Serializable()]
public class Visitor {
public CultureInfo Culture { get; set; }
public List<Guid> Polls { get; set; }
}

Polls is a list of the Polls Guid on which the user has voted.

Instead of storing this here should I store the UserId on each vote
she/he makes? An anonymous user has an ID (Guid) associated to it?

And when the user registers, do I need to migrate this information?
Visitor data is important to anonymous and registered users.

Thanks,

Miguel