This will avoid a delay in you receiving payment or the material being returned. Late bids are returned unopened to the bidder. Since the bids are a matter of public record, anyone may request to review the bids in the City Clerk's office at City Hall. Each 'Invitation For Sealed Bid' has instructions as to the time and place of the bid openings. Although appointments are not required during these hours, making an appointment would benefit both you and the buyer.
Appointments are necessary for other times.