Just a short node to tell you that I am anal enough to test everything as much as can as long as time will allow. Don't pull a MicroSoft and throw it out in Beta form for all to find your bugs. I think that this is unlawful and very annoying (from a users stand point). I also probably test too much. Considering that the code I write usually has something to do with our database, which is the lifeblood of our system, I want to make sure that what I am writing does exactly what I want it to and exactly what I expect it to. In our production environment, if I cause our DB to go haywire, I'm in some serious dudu-caca. (I know, preaching to the choir).
Bottom line from my point of view is that testing is the only way to see what your code is going to do for sure. Just make sure that you don't run over your time budget for your project.
Just thinking that maybe the writers basic rule of thumb isn't such a bad idea. That (as far as I recall) is to put in as much time necessary to complete the book (code), then put in another 10% on top of that improving it.