This is an interesting subject.
The first thing you should know is that the BSA has no official status, so you may safely refuse to undergo a software audit. The organisation represents the interests of various software companies - chief among them being Microsoft.
We're all for software licensing compliance, but the BSA has come in for some serious criticism for the way in which it uses bullying tactics to get companies to undergo a software audit. It is also well-known for its practice of offering financial inducements to employees who blow the whistle on their employers for using unlicensed software.
The simple fact is, you are under no obligation whatsoever to accede to the demands of the BSA, and you might consider responding to the organisation along the lines of 'thank you for your interest, but we are fully compliant,and have no intention of undergoing an audit'.
It's worth underlining the importance of maintaining careful records of software licences, including the retention of invoices, and license slips from software boxes, etc.