I wouldn't necessarily say "playing god", but we do vaccinate ourselves right? I mean I think the real issue is inflicting a change to a being that will become "Sentient" or "Conscious". When we genetically alter that which has potential for "life", do we know, beyond a doubt, that the change we inflict will be positive for the individual AND positive our species as a whole? If we cannot say for certain, then genetically altering sentient life (who have no choice), is not a good idea.