(and i find it frustrating that there's always someone - apparently with no experience - who suggests the very worst when people mention this)
really, this is the height of technical cowardice. searching out exceptions for what is not only a good cause, but one that is statistically safe.
it happens regularly when tor is mentioned. there seems to be a certain kind of person that takes delight in looking for extreme examples to justify their own lack of moral fibre.
Also, it doesn't matter if there's a one in a million chance that running a Tor exit will get you raided for child porn -- the vast majority of people will not take that risk to help others remain anonymous on the internet.
I would be very surprised if you've run a Tor exit node for a considerable period of time without incident, even if it's just abuse notices from your upstream provider.