And DO check the results of the audit. Many of these auditors and not the best and brightest, and certainly they're not infallible.
Biggest problems with audits include misclassifcation of employees, like clerical and non-clerical (clerical premiums are cheaper), officer payroll, etc. This can result in some serious backbilling. Experience mods are another biggie.
Often you'll find whatever you pay your CPA to have the audit done in HIS/HER office will be more than made up in savings. Besides, you don't want any auditor, whether from your insurance company or the state in your office or premises... no good can possibly come from it.