Here's a true story from the #python IRC channel, a frequently-asked question played out for real. I'm nedbat, the other nick has been changed to protect the innocent:
other: there's no way to automagically omit coveraging the actual test modules is there?
nedbat: you can add an omit line to the .coveragerc, but I've found useful info from coverage on my tests.
other: What kind of useful info?
nedbat: useful info: two test methods with the same name (by accident) so one was never called.
other: hah oh boy, one of the test modules I was ignoring sure enough was a test that wasn't being run :). Perhaps I better not, I hadn't thought of that before.
nedbat: yay coverage! :)
other: yay coverage!
Moral of the story: if you have enough tests to run coverage, then your tests are a serious part of your product. You should run coverage on them, it will help.