It seems to me that channel locks are the only way to be safe anyway. I just locked down my system all the way to PG, and I was able to tune into Soap Operas, CSI, an Alanis Morrisette concert, "The Flesh Eaters" on Monsters HD, Jerry Springer, Benny Hill, Fear Factor, ect.
The ratings are applied inconsistently and unfairly as well. On Comedy Central, South Park gets an AO, but not Chappelle's Show or anything else. The only things that would get filtered out on Comedy Central are theatrical movies, but they are so heavily censored, they are probably the cleanest thing they air.
Likewise, BBC America only gives an AO ratings to Footballers Wives, even though they feature many many programs that would get a TV-MA in the US. In fact, they allow "swears" that are bleeped on the "Adults Only" South Park.
And the AO rating seems too harsh for many of these programs anyway. Cartoon Network's "Moral Oral" may be tasteless, but it certainly doesn't deserve to be on the same level as hardcore porn.