Tim:
I've always considered having to check your spam for false positives to make having anti-spam filtering a waste of time.
Heinz Diehl:
It depends. I've been receiving about 30 spam emails daily, on average. A quick look into my spam-folder is enough to check if any serious email accidently got classified as spam.
I'm using a combination of procmail and CRM-114, by the way..
But you don't trust it enough, not to check...
If I had to check up on it, I don't consider it trustworthy. And, probably more to the point, it's an extreme annoyance when you email someone, and their crappy anti-spam software falsely classifies your email as spam. Eventually you give up trying to get a reply from them, and have to phone them for a response.