You will need to set the Artistes rate before you can sign an artiste out.
You do not, however, have to set meals, supplements or travel and this can be done or further changes can be made up until the point in which you send the day to accounts.
Please note that Artistes will be sent a summary of their payment 24 hours after you close the day even if you haven't sent the day to accounts yet. Therefore you should aim to make sure that everything is added and correct before this point.