I think in order to get a fair response from the users, it's important not to sway the question or voting system in any way, in particular any that may evoke an emotional response from the way the question is presented.
Not only does this potentially sway results in a way that cannot be measured, but it also affects responses in an unequal way depending on the question, and indeed the current mood of the user. For example a user who is particularly unhappy is going to pick the crossest smiley as it most closely represents their mood, and a happy customer will pick the happiest looking option. And so maybe you're actually likely to even further promote the 'extreme response'. Similarly for those who prefer to pick a neutral response you're even further nudging them towards the central option because that has an ambivalent face on it.
When gathering statistics, it's totally pointless to try and sway results. It becomes a lie. You'll never know what the response would have been if you'd presented it fairly. It's no use to anyone. It's one thing to misuse/abuse statistics after gathering but to abuse the gathering in the first place...
Stick to stars - it carries no baggage and directly maps to the rating level itself.