This is one of the reasons I really like property-based testing à la QuickCheck[1]. The core idea is simple: you come up with an invariant for your code and the testing framework checks this invariant with randomly generated inputs. So when you're writing a test, you just have to come up with interesting invariants; you do not have to guess which inputs are edge cases.

[1]: http://www.haskell.org/haskellwiki/Introduction_to_QuickChec...

This may sound complicated and seem like overkill, but I've actually found it easier to use this style of tests for much of my code. Having randomly generated inputs can help find edge cases I did not even consider when writing the code in the first place--this addresses the bias issue you are worried about.

I also find that this style of test results in concise, easy to read tests. The usual "hello world" example uses the reverse function for lists. We want to ensure that for all lists, reversing it twice does nothing. The test would look like this:

    prop_reverseTwice ls = reverse (reverse ls) == ls

It's very easy to tell what invariant you're testing for! (The prop_* name is a convention for this sort of test.)

While real invariants are often more complex, I find they are still usually easy to read right from the code for the test.

Anyhow: you should really try QuickCheck or something like it for your language of choice.