While I'm not an auditor, I've been the go-to Excel person at all my internships and in full time roles. I've reviewed, enhanced, and fixed hundreds for tons of different functions (HR, supply chain, manufacturing production reporting). You can build a respectable system in Excel which has built in error flags that make it obvious to the end user. Here are some of my tips for people who deal with spreadsheets:
* Cell styles, much like headings in Word, are really great but nobody uses them. They clue end users about what cells do. If my end users see an orange cell, they know it's an input they can modify.
* Lock sheets, even without a password. 90% of the time will prevent people from breaking the spreadsheet in the first case.
* Stay up on new functions! =Unique, filter, sort, Switch, textjoin, and {sum|average|min|max}ifs have been added in the last ~18 months and can greatly clean up messy formulas.
* Document stuff like code should be documented. The Explanatory Text style is excellent for more in depth explanations but not visually clogging things up.
* Use tables. Name the tables. =Average(tbl_Production[Widgets Produced]) is self documenting, and the range expands with the table. Range references are notoriously obtuse.
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The European Spreadsheet Risks Interest Group has some great information on best practices.
http://www.eusprig.org/best-practice.htm
Joel Spolsky's "You Suck at Excel" is also eye opening for most.