I let everyone know ahead of time, and I don't think any one of them were ever contacted... maybe schools only reach out if you try to say you did 10,000 hours of volunteering in a month or something.
I only let the people who were involved in large portions know. I highly, highly doubt that anyone will even care to verify if I volunteered 20 hours for this one event, but they might actually check for the volunteer position I have over 500 hours with (but that person also wrote me a letter of rec so I also don't think they'd doubt the validity).