It kinda depends on what's gonna eventually happen with the song.
If you're on a major label and want to be able to sell as many CD's as possible, ya have to keep in mind that to make money, your CD's have to be sold at places like Walmart etc...
If you write controversial lyrics, you'll end up limiting the outlets that'll carry your material.
Ultimately, it's your label that'll decide whether you can write controversial lyrics or not, cause they're the ones paying for it.
If you're signed to an indie label or marketing songs yourself, it's totally up to you what you write. Then all ya have to deal with is the public and the media.