There's no built-in feature for this for now - I've added it to the wish list - but you could build a custom report, as you indicated as well. You can fairly easily create a report that lists machines with uncommon software by:
- Opening the built-in "Software: List of software by computer" report in the Reports tab.
- Hitting the Edit Report button on the left.
- Hitting Save As, to save a copy of the report under a different name.
- Adding the below to the Criteria column of the tblSoftwareUni.softwareName expression.
Not In ('name first common software','name second common software','name third common software')