10 years experience
A lot of developers don't pay attention to this, they just want to ship a product as fast as possible. What happens next? The client test...
A lot of developers don't pay attention to this, they just want to ship a product as fast as possible. What happens next? The client test it out and it's full of bugs, so the developer has to fix everything again.
I personally prefer to use multiple level of testing before delivering a product or considering a task 'done'.
Starting with unit tests, functionality test, integration test, and even automated UI tests! A product should be really clean before handing it to the client.