I strongly feel you should try to add one test in each category. That adds a sanity check and lowers to cost to adding more tests when you really need it.
It's pretty painful to think "oh, this really needs a test, but I haven't got a test suite set up and besides, I don't know to write a test of this kind".
Writing tests for edge cases we see in production is the most valuable thing we do. We use Airbrake to find the bugs, and then we add a test for it, if possible (it's not always possible).
That gives us good confidence that other changes aren't fucking things up. It's also a pretty sane strategy for growing a test suite when you inevitably have some portion of your code which has no tests at all.
It's pretty painful to think "oh, this really needs a test, but I haven't got a test suite set up and besides, I don't know to write a test of this kind".
Writing tests for edge cases we see in production is the most valuable thing we do. We use Airbrake to find the bugs, and then we add a test for it, if possible (it's not always possible).
That gives us good confidence that other changes aren't fucking things up. It's also a pretty sane strategy for growing a test suite when you inevitably have some portion of your code which has no tests at all.