While there can be analysis paralysis having a clearly defined workflow and writing a test for that flow before you even implemented that feature worked great for us.

Writing a functional test at this point helps in understanding the problem space and interaction with the service quite well. And with the functional test in place it is a lot easier to see which other part of the new feature needs to have unit tests in place to make it very stable.

At least that has worked very well for us for a long time now