Help, Help, about B-2 visa

My parents have made an appointment for B-2 in Beijing on June 24. Because their province doesn't have the branch of "Zhongxin"Bank, they have to pay the $100 application fee when they are in Beijing. They plan to be Beijing on June 22, pay the money and fill the DS-156 form there possibly on June 23. But someone told me that we had better fill the DS-156 form earlier. Is it true? or their plan will have no problem for visa application. I think the DS-156 should be pre-processed before see the VO in the embassy. Give me an answer please. Thank you.