My OH and I were pondering this the other day. We had originally planned to send one to everyone on the guest list, but then I thought: hang on, if I'd received a StD and then only got an invitation to the evening do, I might be a bit miffed. So now I'm thinking that we should just send/give StDs to people who are definitely invited to the whole day (because there are one or two people who might get shunted to evening-only after we've reviewed the guest list).
Who did you send Save the Dates to?