Write just enough tests at the level where it catches most of your regressions...make sure you didn't make a stupid off-by-one mistake.
One thing that I've seen in inexperienced coders (including myself in the past) is that they tend to think of every bug as a fluke one-off mistake in an otherwise mostly flawless and awesome record. New coders tend to want to just fix a bug, then pretend it didn't happen.
This is exactly the wrong attitude to take. As a discipline, we programmers should be studying our mistakes and taking steps to prevent them in the future. As a craftsperson striving to improve, each of us should be studying our own mistakes and taking steps to prevent them in the future.