Bear in mind that although you have the burden of proof, that does not mean that you are compelled to show every document to the government agent - some documents are simply none of their business. Some audits are conducted by mail, some require you to go to the government agency's office, and other audits require that the government's agent go out and meet with you at your home or business.
Some audits are relatively routine matters.
Many audits result in no change - the required information on the return was accurate and there is no change in the amount of tax. While some audits do result in an assessment, some audits actually result in a refund. Whether you need outside assistance with an audit depends on your particular circumstances. If your tax matters are simple, you may be able to handle it yourself. But if your tax matters are complex or if you don't fully understand taxes, you should get a professional to assist you.