I learned recently that scheduled emails are sent to query results at the time of scheduling rather than the time of sending. 

I think it would be helpful to have the email be sent to the most recent results of the query (at the time of sending) rather than at the time of scheduling. 

We use etapestry to manage event attendance, so we have an understanding of how our donors are engaged. We also use it for all out email communications ie. newsletters. 

We do out best to segment, and one way we like to do this is to not bombard people who have already registered for an event with "Register Now" emails. So we set up compound queries to exclude people registered for an event. As a communications person, I get these emails developed, approved and scheduled before they are sent, usually a couple days in advance. 

This means, I may update constituent accounts with their registration AFTER I schedule out an email. Which means even though they are registered, they will receive the email that they are not supposed to!