I can't help but toss a few throw pillows on my sofa. It just makes it easier to relax. Since my sofa is fairly modern, and lacks overstuffing, throw pillows are definitely needed, in my opinion.
Other, plusher sofas, though, can often get by without extraneous throw pillows. I know plenty of guys' sofas that observe a strict no-throw-pillow policy, but most of the women I know love to decorate with throw pillows. What about you?