Before I sold my hosting company, part of the evaluation included exports to Excel etc. While they only required the previous 2 years in the export, we had a full export since our startup back in 95/96.
If someone came to me today and needed to know what invoice XXX was, or an export of all purchases by a particular client - I'm pretty sure I could get that export. Also had quarterly exports of every account on each sever (CSV export from WHM).
Programs are nice, but Excel and Google Docs can be quite powerful (for record keeping) - I've always used other software for accounting.