Some are certainly quieter than others: generally the ones who don't wear heavy armour, like Brelyna Maryon or Aela the Huntress. They're still pretty poor for sneak attacks though, because as soon as an enemy turns hostile (i.e. when they appear as a red dot on the compass, and they're looking for you), the follower will rush out to attack them, which kind of defeats the point (also, Brelyna, you're a *mage*, stop trying to tank). If you're just trying to get off an opening salvo from stealth, then they should be ok though.