Not necessarily. Games like Minecraft and To the Moon have done just fine on their own. I'm not arguing that they'd most likely do infinitely better on Steam, but they still made profit.
I suppose it depends on what you want your market to be. If you want big, mainstream-indie success like Braid or Limbo, you're gonna want those big audiences of dedicated market places. But those reeeeealy small indie games can still survive without it.