[Blind-sysadmins] Active Directory site/domain design

Hi all, In a multiple site design, is it necessary or recommended to have a separate domain for each site as a child of the primary domain? Or is Active Directory Sites and Services enough for site separation, or does it depend on the purpose for the site design? I'm just curious what everyone thinks, because our setup has a domain for each site, and I'm trying to see if we can possibly cut that down to a single domain, since we're all part of the same project, just different locations, and would it not be possible to configure which domain controller a computer logs onto via locale alone, or does it have to be done via domain? I've not done this in a while, so I'm asking the experts. Thanks.