I would set an appointment, especially if there's a lot in your quorum. That way, the baptismal coordinator will know if he needs to have more workers in the baptistry, and your quorum wont have to wait as long.
The Appointment will be better for everyone. Once I went with my family without an appointment and had to wait for more than an hour.
Making appointments helps both the, temple and you. If you have an appointment, it will be honored even if it's very busy.
Appointments are preferred, but you can show up and they'll try to fit you in (during normal baptismal hours).
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