If this were asked of a planet in a Sci-Fi novel, with it's own non-Terran life and ecosystem, then I think you'd get a better response.

In reply to Venus - I don't think it would be wrong, but it'd be a completely daft thing to do, and the cost involved would be ludicrous.

If it was done to every single discovered planet out there, then it could start to border on wrong. Sure, even if it were done to every single planet that has been discovered up to this present day, it wouldn't even begin to start to scratch the surface of what's in the Universe, but I'd still consider it as damaging the natural beauty of the Universe. For all we know, no two bodies are the same, and by terraforming one we just happen to come across, we'd be destroying something unique.