Stay afloat, don't sink
The practice of writing unit tests for classes creates a safeguard against the possibility of sabotaging classes after creation by way of changes or the introduction of conflicting logic in other classes. The TTD practice is a safeguard. It is the life preserver which allows Agile teams to work with their heads above water.
Headspring Systems offers Agile training courses which teach Test-Driven Development. We are Headquartered in Texas (at Austin) and host our trainings there. Our Agile Boot Camp™ offers the best place for one to get started by way of an introduction to Agile/Extreme Programming/Test-Driven Development. The class details the advanced practices of experienced programmers.